First, there's no reason to assume the bonus only applies if you can see the clothes.
Except for that your clothes have to be visible to give you the boost.
Still presents a problem though.
Following is a list of things that I believe can have their limit augmented by simply wearing nice clothing (which I admit
is very nice clothing):
- Banishing.
- Creating foci.
-Disenchanting and disjoining.
- Brute forcing your way through a barrier.
-Spell defence and dispelling.
-An argument could be made that assensing targets on the physical plane would get the bonus.
While I can understand that looking nice might increase your confidence and all, it does balance things a fair bit towards Charisma-based spellcasters. (They also can have their limit augmented by smelling good, but that's balanced somewhat by an essence cost, and logic-based mages have the mnemonic enhancer for a higher mental limit as well).