Can mods be stackable?

Just spitballing, but it seems totally feasible in concept (if not in execution) that you could sandwich a battery mod between the Z and another mod (say, a sliding physical QWERTY). It would just have to be double-faced, rather than having a closed back. And then if you don't have 2nd mod, just smack a style-shell on the back like you would on the bare device. Bingo-bango.

Or is that concept a total non-starter for some reason I'm blithely unaware of?