Over on Monsters & Manuals, noisms dug some optional per-class xp rules out of the AD&D 2e DMG:
Fighters get XP for “defeating” enemies
Priests get XP for successfully using their powers, casting spells to further their ethos, and making stuff
Wizards get XP for casting spells to overcome “foes or problems”, researching things and making stuff
Rogues get XP for using their special abilities and, er, treasure
This sounds similar to (and was probably a direct ancestor of) “Keys” in the Shadow of Yesterday. There’s a pdf of that at http://downloads.darkon.info/pdf/tsoy.pdf — the list of standard Keys starts on p27.
I’ve had good times with Keys, but I’ve only used them in the kind of games (like tSoY) where we leap from scene to scene, resolving big conflicts in a single roll, and so forth. In a turns-and-rooms D&D game it would be harder to make them sing.
The Wrong Kind of Wax 
You probably know this already, but I made a supplement for Dungeon World (and other d&d types) with Keys by class:
https://docs.google.com/document/d/1TMtedwe7X_CZ6K07rdJfGR904RgamtFJYOXLUaia87E/edit?usp=drivesdk
No, I hadn’t seen that before actually.
That’s an interesting idea, and something I’ll consider if I run DW again. Not sure about buyoffs, though — in TSoY they have a specific role (give you enough xp to immediately buy a replacement key). I don’t think that’s needed here?