Exactly which drives you can install, I don't know about. I would guess that just about anything would work (like the car player), but I'm not sure. Empeg guys, any comment on this?
We haven't tried it, and it would void your warranty, but in theory any IDE drive should work. Cooling is the only potential problem. Oh, and I'm not sure when bigger-than-128Gb support went into the Linux kernel; we use 2.4.13-ac8.
Unlike car players, you can't add a second drive (there's only one IDE channel, and its slave position is occupied by the CD).
Those proprietary protocols are well-documented (they were reverse-engineered for the Rio Receiver) so you could write your own receiver application
Someone really needs to write a Receiver client plugin for Winamp. Someone's needed to for a while, in fact -- but someone really needs to now the audio centre is released.
Peter