This pinout is took a while to complete. As part of an effort to connect a Teradrive to a SegaCD unit, I had to trace the expansion port. This is it. Some pins are labelled by destination rather than signal, for example 87/315-5364. This means it runs to pin 87 of the IC 315-5364. Also, this refers only to the model 1 Genesis/MegaDrive. There may be minor differences in the model 2, for example the audio in the 2 does not require the patchcord, so I assume there's a small difference there.
Some of the pin names may sound a little odd, they're based on corresponding cartridge port pins. The majority of the pins on the expansion port, for example the data and address lines, connect to the same pins on the cartridge port. /C_OE is Cartridge Output Enable, which sounds a little odd on the expansion port, but I'm sure you'll adjust. If you have any additional info on this or the model 2 differences, please email me.