Mm, perhaps it would be best to only make light crossbows universal, and leave the more specialized heavy crossbows restricted. That is what my original mod did if I recall correctly. It should not be hard to recreate, just check if it's a crossbow or bolt, and if a thief can use it. If yes to both, allow a mage to use it as well. Clerics should probably still be restricted due to the ban on edged and piercing weapons, and IMHO such a mechanical weapon designed for hunting game doesn't quite fit druids very well either.
Oh what the hell, I'll write it up. :-)
EDIT: And done! Here is my version.
https://github.com/AngelGryph/MadeInHeaven/blob/development/mih/components/versatile_light_crossbows.tpa
(If you want to try my 'Made in Heaven' mod, please feel free. But do keep in mind that it's still in full development and probably has lots of bugs still.)
I've recently started playing BG1 with your Made in Heaven mod installed, and I really like it - tweaked encounters look very promising (and that one quest made me laugh). I'm just wondering if you have any host/forum for your mod beside GitHub. I tried googling it but couldn't find anything...