I think this idea is terrible. We had enchanter rings. They were sick and retarded. Even if you restrict them such that they only allow boosts to what the user currently has access to I think they're still going to be horrid to effectively balance.
I certainly would be opposed to a whole guild being based on this one mechanic - it's basically a doppleganger guild if you allow any skill/spell/effect to be boosted, and if you can only borrow prepared skills/spells/effects then it's an actual revisit of a single spell in the enchanter guild a long time ago.
Now that brain-eating guild idea would be cool, if you could preserve brains and eat them later for bonuses or extra abilities...
