Basically you counter the problem one way with effort (fixing it or casting the spell), but then saying it can become easy because they can simply use a version of the spell with group, etc. All you're really doing is making the point that if it's important to someone, they will make it available. But that don't mean it was easy. the person that collects and casts the spell did it only after getting their Arts high enough and learning/inventing the spell (or high enough to cast it spontaneously) and then painstakingly gathering ACs (on the sly most likely) and then enjoying the fruits of their efforts. To be honest, even if they come up with an efficient method of doing all that, doesn't mean it was easy. i say that at my job all the time; "I might make it look easy, that doesn't mean it was easy". To the magi that didn't go through all that effort, yeah, the end product looks like a cheap end game maneuver, but that doesn't mean it was actually easy to accomplish.
Take that info and then present it like this: Now that you've seen the power this provides, know that you can do the same, if it's important to you. And if it's important to you, you can also take steps to prevent it.
Then finally is the social stigma. I definitely think this can have a large impact on people's style and methods. On the low end the person collecting ACs will likely get a bad reputation. Continuing the activity will worsen it, likely bring charges (founded or not), investigations, Wizard Wars (either by the people he's gathered ACs from, others that are equally paranoid, or hired muscle). Being snubbed or targetted at Tribunals for the actvity and finally leading some to perform research, Mysteries, etc that can stymie the AC collection business. Repercussions exist, and of course any method they use can be copied by others and do the same thing to him. Even if the person takes precautions to prevent them from gathering his ACs, that will lieky be inconvenient for him and then others will take precautions and now AC collecting is neither easy, guaranteed or even safe. One could always make the argument that the AC is part of their magical power, whether the argument sticks or not doesn't matter.
Keep in mind that just because you collect say all the hairs you can find having fallen in say the eating hall of a covenant does not mean that the target you have in mid will have lost a hair on that particular day in that particular location, and if maids are cleaning the hall every day or every other day you may be putting in a lot of effort for little payout (most of the hair will probably belong to the cooks or serving staff, since they are there the longest). Additionally it becomes very difficult to be circumspect about these mass collections, and if it is another magus you are looking for an AC to you are likely to be charged for scrying on them (technically using an InCo on an AC would be scrying against another magus)
It's rather a stretch to call the casting of a spell (the InCo one, even) to determine who is the target of an AC. The spell shows absolutely nothing about the individual's current circumstance, except, perhaps, that he is alive. It also doesn't affect the target (which is one reason I dislike the spell), since it doesn't penetrate magic resistance. But even then, there's no way to know when the target of the AC has been scried upon.
If it goes to tribunal because they are sifting through a bunch of unknown hairs to find who they belonged to, I really doubt that issues of being able to prove specifically whom he has scried upon will be the main issue compared to general outrage that he has been collecting ACs for what seems like the entire covenant.
Now collecting hairs at a noble's banquet hall could be an entirely different issue- there you are more likely to be either summarily executed or- if you are seeking an AC to someone the noble dislikes, possibly supported.
How does anyone but the magus and the servant collecting the material know? If it's a matter within a covenant, well, the magus gets kicked out of the covenant when it becomes known. It isn't fait accompli that it goes to Tribunal. You have to prove that there was scrying or that he peered into the affairs of a magus. Matters of fact are discussed before a Presiding Quaesitor of the Tribunal before this makes it to Tribunal gathering. It's unlikely that this goes beyond that step without some powerful evidence.
The InCo spell in Houses of Hermes: True Lineages, and the spell presented here, which I think is better, does not scry upon the target of the AC, nor does it peer into another magus's affairs. Possessing an AC to another magus is not, in and of itself, a crime. Using it in an attack, or to actually scry upon the target is the crime, and that hasn't happened here.
It might be too late for that then. Time to speedily consult your trusted champions for certamen or wizard's war?
See for the typical implications of searching an AC to a magus without his consent also A&A p. 72 The Genethlialogical Investigation: "The caster learns the target's place and time of birth. Casting this spell on a magus is tantamount to declaring Wizard's War, since there is no honest reason for needing this information without the consent of the target."
Because if you are using this method you are spending hours collecting and cataloguing hairs found lying around in public areas, which is frankly rather odd behavior even for a magus? This approach lacks a certain something in the subtlety department that means it is bound to be noticed...
Magus: After you've swept up after the evening meal for our guests tonight, I want you to sift through the detritus and bring me all the hairs, to my sanctum, and tell no one and make sure you aren't witnessed
Grog: Yes, magus.
Unless the grog is seen sifting through the garbage, no one is going to know what's going on. If someone does see the grog doing this, and he is asked about it, unless the person asking ranks higher than the magus who gave him the Order, there's going to be some evasion at best. But, this would not be asked of a dumb Grog, either. It's something a trusted grog would be given, and he would likely be well rewarded for his continued loyalty. The only work the magus will actually do is cataloging the hairs, in the privacy of his sanctum. If someone discovers what he's doing in his sanctum, well, the magus definitely has a claim of someone peering into his affairs.
You would need to forge a very strong bond with that grog, both for him to not comment on the oddness of he request to anyone else and to accomplish it reliably as opposed to taking shortcuts of some kind that would be less effective. If this is the case it is likely that someone else will know where that grog's loyalties lie if they see him picking through refuse for hair.
No, you just have a stronger bond with the grog than any other magus might. Someone who might want to be a shield grog, or hold some other position of importance or status.
Eh, people talk. Sure you can instruct the grog not to tell anyone about it, but covenfolk aren't a modern military used to sealed orders and discipline. He could sift through some of the trash, go down to where a couple of friends are hanging out, mix in some of his own hair and theirs while complaining about what a strange task this mage has set him on. If you want it done right and quietly, you need someone loyal. Not saying it won't work, just that it isn't as simple and straightforward as it might seem at first glance.
Then there's Perdo Mentem magics for the grogs you can't trust. And Intellego Mentem to see who else might know from the grog who couldn't keep his mouth shut.
(As the OP), I personally like Johnathan.Link's ideas that ACs have to be "harvested" collected or taken with the target known and with some sort of Intention. ACs are a way around the Limit of Perception (or what ever the targeting Limit is called), and by collecting a known AC it makes that connection. Just collecting all the hairs left over after a banquet seems like a good process for looking for a breakthrough on how to extend AC limits.
This actually does not really answer my OP question, but makes it clearer how I want to proceed.
The spell should be InIm to begin with (as designed above it would create the image near the Arcane Connection's target). Momentary duration would be enough to give you a flash of the thing it is linked to and lower the level to 5. If you want to project this image add a magnitude for a Creo requisite and one for an appropriate duration (I'd suggest Concentration), bringing it to 15.
It needs some polish. I've also considered that having a proxy collect the AC might make it weaker by a level or two. What that change would do is force the magus to either spend time in the lab to fix or use the AC almost immediately, or actually go through the effort (story) of collecting the AC himself.
Time to speedily consult your trusted champions for certamen or wizard's war?