Theoretically, you may be able to wire one up internally. I'm pretty sure that the internal lcd, and keypad use the same transmission medium as external remotes. It's probably a long shot, but if you have some soldering knowledge, and some pinouts, it may be possible to wire up a remote.
Id try it with my MZ-R500, but I dont really want a remote at this point :wink: . Maybe if I feel the need later on il try.
Edit: Open up the battery compartment, you sill notice 4 copper pads inside the minidisc player. Ive heard that these are points that are used in the remotes, so a remote may not be all that long of a shot after all.