One way would be to check if button 65 is present (ASCII 'A') and if so set the appropriate symbol on each button in a backward compatible way.  As you have the ascii ranges 65-91 and 48-57 to play with (36 keys) you could fill them pretty easily from the ABC English map by mapping 48-57 to 100-109, 65-73 to 120-128 etc.

My concern is that we need to give skinners a bit of time to adjust - there'll be a bunch of changes going into the skin now that Gotham is finalised, so allowing them to incrementally work on things would be useful, and I think it's achievable here without much code?