Long story short, I want to make something of an "influence others through art" type of Magus. Assuming the Magus is not themselves present (that would make this exceptionally easy, because Deft Mentem) how would you make art (of any kind) that magically compels people to think or act a certain way while being inconspicuous about it? A painting that inspires longing for freedom upon being seen, or a poem that compels people to revolt against nearby authority after hearing another mundane read it to them... Well, the second one actually doesn't work within Hermetic limits, so ignore it I guess. But the first one shouldn't be impossible. I guess you'd use Linked Trigger and instill an effect allowing the item to see? Then the aforementioned painting could activate a Sight-range spell in response to someone looking at it. Does that work? And what other ideas can you guys think of for this kind of thing?
Enchanting Ability...
Alternately: crazy-insane high Artistic ability (Poetry, Painting, whatever), and just treat that as enough. Great works of art HAVE done that, (in a not-quite-so-cinematic way) in the past.
Or to answer the question a bit more directly:
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The target would likely be some sort of variation of the Merinitia Bloodline(?) target - only instead of "all individuals of a lineage", it could be something like "all physical copies of this poem."
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The Range could be a Perception(?) magic: "anyone who views this piece of artwork" (Sight), or "anyone who hears this poem being read" (hearing). I forget what they're called, but they're described in HoH:MC.
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The duration would be tricky - again, it would probably be something Farie-based. "Until the rightful king is put back on the throne" - for the revolution-poem, for example.
Not sure if those would ACTUALLY work (serf's Parma), but it may be a good place to start kitbashing your own version together.
Perception Magic is an interesting idea, though it's sadly limited to Bjornaer magi. Mixing Faerie influences in with that would be hard and likely draw flak from fellow Bjornaer magi, but it's an interesting idea... I believe one of the Faerie durations is "Until (Condition)" which would certainly be helpful, though it might not work as intended since the condition is supposed to be part of the target, and it might be a stretch to say "until the target of the spell is aware that one of (so and so bloodline) is upon the throne". But it's an interesting area of research.
Enchanting Ability doesn't work for these purposes and fall into the category I mentioned earlier; controlling people's feelings is hilariously easy if the Maga in question is present. Enchanting Ability is about the person doing the thing, not the item they do the thing with, and it won't work without the person there.
Surely a Verditius with the Items of Quality Inner Mystery could combine a good handsaw or chisel with a sculpting-related Craft ability to make an extremely good sculpture, and that might be able to mundanely influence people? But that's more akin to Rego Mentem rather than Creo or Muto Mentem, strengthening their feelings and nurturing their doubts rather than really cramming new ideas irresistibly into their heads the way magic does it. It might work to make peasants revolt against a mediocre noble, but procuring the loyalty of a proud, paranoid noble with such a "gift" may be beyond the means of such a thing. Still, workable to a degree.
I'm really looking to do this with magic items, rather than using magic on-site or just making really good mundane stuff and hoping for the best, so thank you for the D/R/T ideas Kevin!
What about an effect triggered by an InMe effect with T: Room? If it detects anyone new in the room it casts the spell on the person. Perhaps the spell would have R: Eye and the target would have to look at the art.
Wouldn't you have to give the thing in question eyes for that to work...? Well, now, that's true art! 
Regarding a Bjorner reaction - from what I recall of the section where they discuss it in HoH:MC, it suggests that the main reason it's still part of the mystery cult is that it was on Bonisagus' "To-Do" list, but he didn't get around to it. (Unlike normal Hermetic perception magic, which he did integrate.)
Things may have changed since the Founder's time: what was once something being given freely to the Order has may have been codified as belonging solely to them. Were I running it, though - if a magi were to try integrating Bjorner perception magic, the Bjorner would probably give a collective shrug, say "OK. Good job. Glad that's finally done," and then go back to doing their own thing.
I suppose this depends on how you run each of the various Mystery Cults, of course.
Hey to burst your bubbles, guys, but Sensory Magic (the Bjornaer Inner Mystery) cannot be used that way. It explicitly mentions both that it can't be invested in items and that the magus is always at the center of the effect. Would be kind of cool, but it goes against the House's flavor.
Yeah, that sounds right - I seem to recall something about requiring that the target having to be Personal. Ah, well. That being said, it's an avenue for Original Research, if nothing else.
Well, Personal range effects can still be enchanted into items, so that particular limitation isn't actually an issue as far as items in general are concerned, though I guess that would put a damper on the way the thread starter wants to use the ability.
Also, Kevin, this is a bit off-topic, but do you usually play Bonisagus magi? You really seem into the whole "research to improve Hermetic Theory" thing.
Irrelevant. Sensory Magic explicitly states that these effects cannot be invested into items. HoH: MC, p. 28, first column, second-to-last bulletpoint.
My preference is probably T: Room, but it depends a bit on the specific piece of art.
I didn't feel the need to mention that, as I already said that very same thing in my previous post. I just meant that Kevin's point about the spells being Personal range didn't matter.
Ah. My apologies then.
If all you want is a character who can do it, take Enchanting (for the painting) and possibly combine Enchanting with Personal Vis source and Craft magic (the non-hermetic Major Virtue of Rusticani from HoH:S) or Lesser Craft Magic (Minor Virtue from Rival Magic). Enchanting subjects anyone who witnesses your performance to "emotional manipulation". Craft magic, greater or lesser, allows you to enchant items naturally containing vis to obtain objects that give their users (a specific) use of a Virtue you possess - e.g., if you have enchanting music, you can Craft a magical hide containing vis into a drum that strikes panic in those who hear it. It's kind of tricky to have an item containing vis that you can turn into a poem, but Personal Vis source could give you access to that (your poems contain vis!). Note that the total cost of Lesser Craft Magic, Personal Vis Source, and Enchanting is only three Minor Virtues, so you could easily "fit" them to a magus (or a companion). Perhaps the magus' goal is then to Integrate them into Hermetic magic!
Now, if you want to achieve the same effects with vanilla Hermetic magic, it could get much harder.
For the painting, as you said, you need two effects: one to detect that the painting is being watched, and one to impose the emotion. I'd make the first based on Hearing: InMe 44 (Base 15: is the target watchinng me? R:Per (+0 magnitudes) D:Sun(+2 magnitudes) T:Hear(+3 magnitudes), constant effect (+4levels : 2uses/day, linked trigger). The second is of course the Mentem effect you want.
For the poem, uh-hum. It could be done, I guess, but it seems highly impractical. You could write the poem in a book that constantly "listened" on the thoughts of the people a) around itself and b) around the targets it had opened an intangible tunnel to. The book would then (linked trigger) open a new intangible tunnel to whomever read or listened to the poem, and to whatever object the poem was being recorded on; and it would constantly renew its tunnels with environmental triggers. It could then affect the "tunneled" minds with the Mentem effect. Ugly!
A more elegant approach, that would require Faerie magic and some Storyguide discretion, would be a MuIm(Me) effect that changed the auditory species of the poem into "unnatural" species causing emotion (after all, you can change water into a liquid giving you hallucinations, see the Aquam guidelines); with Bloodline target it would affect all the subsequent recitals of the poem (the "descendants" of the original recital). As I said, both the "unnatural species" and the "descendants of a poem" would require some Storyguide discretion, but if I were a Storyguide, I'd probably allow it, with the stipulation that the strength and type of emotion be tied to the quality and tone of the poem (you need a beautiful, sad poem to inspire great sadness).
I usually play a "pro from dover" style character, who is really, really good at something - but that's a general play style, rather than anything specific to AM. It just so happened that in my most recent AM saga, I played a researcher (Ex Misc, rather than Bonisagus, though) - so, I was heavily involved in the research/integration rules.
That being said, I do like the game mechanic of "let's change how the magic system works, in-universe". Also, prior to this, I played HERO system, which is pretty up-front about designing game mechanics to suit your needs, rather than taking exising mechanics to shore-horn in concepts.
The reason I brought it (Original Research and Integration) up here is that, as stated, I don't think Hermetic Magic can really pull off what the OP is asking - but there are some game mechanics that can get it close. Kinda. But if you can get it close, and if you understand the general consequences of what will happen if you DO fully implement the change - then there shouldn't be a problem.
Of course, it's helped by the fact that AM makes no strong attempt to be balanced (unlike HERO, which at least has some vague gestures towards that. Sort of.) As such, it's really about "what kind of stories will this allow us to tell in Mythic Europe?" rather than "what kind of point balancing will this cause?"
The type of magic described in the OP sounds exactly like what Maestros do (from Art & Academe). More specifically, the bards of power described in The Contested Isle can work this kind of magic using R: Per, D: Performance, T: Sound. Yes, this uses the Sensory Magic Target of Sound in an 'enchantment', but it is not Hermetic magic producing the effect.
Mark
I'm a bit confused about the suggestions for Enchanting (Ability). As far as I'm aware, it requires the Magus' presence as the effect is centered on the Magus rather than his tools or finished products (for example, an Enchanting Playwright's written plays won't cause the enchantment)... And even worse, a strict reading of the rules gives the enchanting ability on account of the skill itself being performed, so, say, a painter, would enchant people who watch him paint, rather than those who see the completed product.
The Sensory Magic limit is unfortunate, but makes some sense. I was always sad about the Bjornaer and Criamon sections of HoH since they didn't give members any ways to expand their capabilities or take new niches, instead just getting stronger or more specific versions of abilities they already had. But that's neither here nor there.
Maestros... I haven't read A&A, but I could check that out. It's still subject to my previous inquiry about Enchanting (Ability), though.
Faerie Magic seems to have the most potential of anything yet mentioned, followed closely by using the Research and Integration rules to make Sensory Magic an enchant-able thing. Thanks for the ideas, everyone!
That's unclear. It seems that you must hear the music to be affected, so the magic is in the music.
As for the poem, you are crafting (using Lesser Craft Magic, or a variant thereof that works on immaterial stuff) a poem that any "user" can "use" (i.e. recite) to produce a given Enchanting Poetry effect - project a given emotion instilled at the time of the crafting.
Then Enchanting Poetry would work while people watch the poet compose his stuff rather then when he recites it?
It seems to me that it's never stated clearly, so it's really up to you. In my saga, we had a cook with Enchanted Cooking, that could move to tears (of joy!) those who ate her food.