In case anyone's interested, here's what I've found out so far by examining a broken CD reader (marked ENR-007, also EMW10447 - 006E, probably a run/revision number) from a "Model 1" Saturn:
- The CD reader control seems to be the responsibility of a standard Hitachi (surprise!) H8 microcontroller, which seems to be connected to a 16K x 8 SRAM. The H8's internal ROM may house custom stuff for reading the security code. The part number of the H8 MCU is HD6433712H; the part number of the SRAM is LH5116NA-10. The H8 is probably responsible for exchanging commands and data with the CD controller on the mainboard.
- There is an 80-pin chip that doesn't have the part number silkscreened, but it is visible on the package. The part number is HD49236FS. This is almost certainly a Hitachi part, but I haven't found any information on it. It may be a semicustom part for Sega.
- Another chip, "5F4 A12178" (again, no part number silkscreen). I haven't found any information on this one either, but from its positioning I would guess that it has something to do with controlling the motors.