As far as I can tell, Roland had some kind of display and keypad support in mind when they designed the CM modules. The CM-32L has connectors that match the connectors for the display and keypad inside the MT-32. They're not identical (I guess that would have been too easy), but close. And one day I'll figure out how to hook up a key panel and LCD display to the CM-32L
Also, I have tried the CM-32L's ROM in the MT-32. It worked, somewhat. The display showed "**** Roland ****" on boot up and then proceeded to the normal channel display. The keys worked, but the volume control didn't. Sound didn't work at all, I got a continuous droning noise, and when I tried sending it a MIDI file it crashed at the SysEx (I think it responded to some display-changing SysEx first. Not 100% sure).
So, not quite compatible modules, but very similar nevertheless.