Zelda Classic, for those who aren't familiar, is a remake of the first Legend of Zelda that also includes a quest editor and the ability to play custom quests. The quest editor's a seperate program that isn't really suited to what Recalbox sets out to do (and I can't see it controlling well with a gamepad anyway), but the game itself would seem to fit if making it work is possible. Unfortunately I don't know enough about the underlying codebase of either Recalbox or ZC to know if it's actually possible, especially considering the possible control complications. I believe every necessary function has at least a keyboard mapping by default, though.