Hiyo,
I've had one potential solution for this requirement in the back of my mind. What I *could* do is externalize a data file which is read in, and allows players to map keys in a table: e.g. {{"A","Z"}, {"B","C"}, {"D", "F"}} etc. This would remain the same between games, and live next to the save file. It would probably utterly break the game if created incorrectly, though, but deleting the file would fix any issues.
I'm not keen to add this as a UI feature in game as it flies in the face of the whole thing. Having Arrow showing up as 'A' but activated by 'Z'? Sounds like a disaster - it's hard enough to get people to leave their comfort zone of their 5 fave spells as it is (Have you tried being a FlyKisser? Or a ShuffleMage? Both are kind of awesomely fun ways to semi-break the game).
Sound like it might do the job? I can offer an alternative super-hacky method if you're up to editing code and just want to see if it's worth it. PM me from the Fluttermind website and I'll go into the grubby details.