Say there is a powerful Verditius archmagus who has decided that their life's achievement will be to create a ring of wizardry that can effectively turn a mundane into a magus. It's not enough for the item to merely let the mundane cast spells -- this Verditius wants to raise the mundane as an apprentice and convince the Order that they are a genuine magus.
Of course, there is greater glory in creating a greater wizard. It is even better if the mundane wizard can go on adventures, attend and vote at Tribunals, survive a Wizard's War, or become an archmagus themselves.
Here are some of my initial ideas of what the ring might need to do or the Verditius might need to arrange:
Item effects to make the wizard appear magical: the phantom gift, masking the odor of magic (to make the item appear mundane)
Item effects to adventure as a wizard: Bind wound, Veil of invisibility, Pilum of fire, Lamp without flame, Gather the essence of the beast (to get vis for trade)
Faking a Familiar/Spontaneous magic: Find a magical animal companion or using the Bind Magical Creatures mystery to bind one with Crafter/Master of (Form) qualities. Not sure how to integrate with an item
Longevity Ritual: Trade vis for one? Or the Verditius builds a luxury home with a great living conditions modifier?
Performing or avoiding Certamen: no clue
What item effects, mysteries, or other methods can this Verditius use to create a magic ring that will let a mundane convincingly play the part of a Hermetic wizard? Or, what are major obstacles that need to be solved?
It would be the most amazing forgery ever performed in the Order.
That said I have no help to offer.
You can easily simulate formulaic spells with items. Spontaneous spells are not possible with an item, but the mages always can claim, that he can't cast spontaneous - that disadvantages is well recorded.
I like the possible political ramifications. If he takes the right measures, this could go unnoticed by quite some time. The greatest liability to such plot would be the impossibility of enchanting a MuVi effect to change the sigil in the items.
Faking the gift: The Phantom Gift wouldn't be necessary if he claims to have the Gentle Gift.
Familiar: No need to bind. Just convince a magical creature to play the part. Use an item with a Cr[In]Me effect at AC range to be able to talk to the creature at distance, this should be enough to convince most magi that you have enchanted the cords.
To fake magical knowledge: Learn a lot of Magical Theory. Pose as a theorist with profound knowledge but weak arts (this also helps because it's easier to have an item with several weak effects).
"Learn" new arts: upgrade magical items little by little.
Lab: every magus goes to the lab once in a while. The master must craft a few normal items and let the magus claim to have been the one to enchant them. The same goes for a few new spells. It is weird if someone with a profound MT doesn't have at least one signature spell.
Keep in mind, such a Verditius, if it's ever discovered, would be easily adopted by House Tytalus, whether he likes it or not...
I recall their being a story seed about a Bosinagus and a Deidne who worked together in obscurity to make a breakthrough that would allow the enchantment of spontaneous effects (with little explanation as to what that means).
There is also the option of meta magic spells that modify other spells, although in my experience to provide any more range in abilities it requires a rather large spell, which comes with added vis costs.
You might be able to feign spell casting ability with those special types of vis mentioned in Realms of Power Magic and a bit of Legerdemain.
The main problem I see is the lack of the Parma Magica.
This could be faked to some degree with relics and similar methods.
Other magi are not affected by the Gift if they (the others) are under the Parma Magica. I assume they simply can't tell if a mage is Gifted, but perhaps this is not so.
Don't forget Second Sight can see the Cords (page 104, main book)
It's only an issue if someone casts a spell at them.
If I was pretending to be a magi, I wouldn't want to be accidentally affected by an apprentice's turbulences in the middle of a tribunal, now would I want to be caught by an intellego vision spell cast forcelessly, breaking my disguise, but hey, risk it if you dare.
Over the long term, the greatest problem that comes to the top of my head is warping.
Your mundane archmagus will be affected by warping the same way mundanes are and so will accrue flaws over time.
Off course they could try to pass those flaws off as twilight flaws, but IMO twilight flaws differ from warping flaws in subtle ways that experts may discover.
That seems more of a Tytalus scheme, who would think it was hilarious, though he might employ a Verditius mage as part of the plan. The Verditius might even win an award at their regular contest once the imposture was exposed - the House respects cunning craft over the dignity of the other Houses, and every year the lie continued undiscovered would simply add to the score.
I seem to remember that Spirit mysteries let you learn "spirit spells" that can cast spontaneous magic.
I can't remember if these can be enchanted, but if so, it could maybe be a solution for that problem.
Hum... Now that you mention that, I think they might already had done that play, but with a fae, and that the result was another house's civil war and taking over by their impostor fae.
Sounds like you’re thinking of the Invoke Spirit of [Spell] and Invoke Spirit of [Form] spells from Hermetic Theurgy. Pretty sure those can not be put into an item. Even if they can it would be obvious that the user of the device was not the caster to anyone using any sort of magic vision or second sight or whatnot.
I'm not sure, null. You could be correct, but I couldn't find the prohibition for investing theurgy spells in an item when I looked. As far as I know, there's no general prohibition to invest mystery effects into items, just case specific ones (like sensory magic).
Edit: I stand corrected
Which means that the next question is..."Can you invoke the spirit using Hermetic Theurgy as normal, and then bind it to an item and still get it to do its thing later (possibly via standard Rego to compel it)?" Plus "Can you stop it departing after one use?"