I'm not sure why you need to do this? A Difficult Art just means that the character's Score in the Art advances when you expend XP on it as if it was an Ability.
If the character reads a book with a quality of 10, then he gets 10 XP. This will result in less of an increase in Score for the character if the Art is Difficult.
If a Summae has as Level of 10, then a character with a Difficult Art can keep on reading it until he reaches level 10. The Difficult Art just means it will take him much longer to get to level 10 than a character who treats the Art as normal.
If a character who treats an Art as Difficult writes a Summa then it will likely be rubbish. But this is just because the character has a lower Score in the Art than an equivalent (same XP) character who treats the Art as normal.
I think you are making things a bit more complex than it needs to be.
I think that I would not let Initiates work in a lab at all. So they can only use Spontaneous effects.
Then you can have additional Initiations (and Virtues) to allow the character access to some or all of the magus' Lab activities.
Hm, I like it, if my ideas spark such an amount of forum activity - especially as my other threads centering around learned magicians don't seem to be of much interest. Thank you to everybody for your contributions, sceptical as well as basically reasuring ones.
I think that in my next saga I will introduce the concept as an ongoing research project, either by nuding a lab rat PC into this direction or (more likely) as the project of an elder NPC. In that case the players will be confronted with the turmoil caused by it.
These will be the technical parameters I plan to use:
If it is a PC's research I think I will stick to calling the base research only a minor breakthrough. Actually I think the whole project most certainly to count as a major one (but not hermetic, can't see any limit of magic broken), but considering the fact that the researcher also must develop a sufficient skill in mystery lore and experiment with initiation scripts, I would still say that is sufficient. Otherwise the character will be kept out of other stories far too long (and this one won't proceed for quite a while). In the case of an NPC this question probably isn't all that relevant.
The techniques will count as major virtues resulting in difficult arts, the forms as minor virtues and normal arts. Actually, considering in time logic, I would feel better with normal arts in any case, but off time this seems more compatible to the power level of hedge arts initiates - and I like streamlined rule-mechanics.
As hedge arts initiates aren't restricted in using the various methods to use their traditions' arts (spells, labs...) (unless these restrictions are the results of their initiation ordeals) I don't think that I will deny much to those hermetic initiates either. Sure, the Parma will never work for unGifted people, and I am doubtful whether certamen might not need some kind of additional virtue, but basically I think the characters should get something worth their money.
One day I will post my experiences with this, but as that saga has not started yet and this story ark won't be the first one, this might take a year or two from now on.
Oh, just before I finish this post: There were a reaction or two to my original presentation of my idea that seemed a bit shocked, and even so I am not entirely sure to which game's style that post refered I think that this was a misinterpretation. I just read the hedge magic rules and did what I mostly did, when working with an interesting new rule book. I fast read the whole book; I thoroughly studied the sections that interested me most (general rules and learned magicians in this case); I used them by constructing a character and some spells based on this background; I searched for some errors or problems in the text (only have posted them partly yet) - and then I started to look for interesting implications not outright stated in the text to present them to this forum. I will freely admit that Ars Magica campaigns I care to play (or organize) should turn out epic in the end and lead to significant changes either within the OoH or within the society of ME or even within the metaphsical setting - or all combined. But as a starting point I really like most of the RAW setting, and any new campaign I would probably not base on the results of a previous one. But I think that this is one of the strengths of the system. You can play a (comparatively) normal live in ME and have fun with it. Just cope with faeries, mundanes and the Dominion, be wary of demonic plots, develop your arts and participate in the political life of the order. There are challenges aplenty for young mages. But there is also much to to for a band of grizzeled archmages, Praecos and Primi. You can just adjust the stakes in game. This story arc (if initiated by players) would probably fall somewhere into the middle.
Well, anyway, anybody should play as they like, I thank you for your comments and wish you a happy new year!