Looking at this discussion, something occurs to me:

Wouldn't it be TONS easier to implement speed-controlled volume as a HARDWARE line-level module that electronically attenuated two pairs of RCA outputs?

You would set the player's volume to the maximum top-speed volume, then adjust the bottom-level (low speed) attenuation via a knob on the hardware module.

Seems like this would be a lot easier than interfacing with the empeg, as you wouldn't need to build any serial-communication hardware, nor would you need to program a PIC with the serial codes. Okay, not as COOL as doing an empeg interface, but it would be more universal, too.