OK, R&D funding found, parts are gettin' ordered very soon. I think I'm going to settle on a nice 4Mbit NVSRAM so I can do 8 banks of save data with no need to ever worry about a battery.
For those who are curious, the NVSRAM acts just like a normal SRAM when powered up, but when power is removed it immediately transfers all the SRAM data into a special onboard non-volatile bank, from which it is restored when power comes back. They can supposedly maintain data integrity for at least 20 years with no battery, even when used intensively.
I'm also settling on 27c020 chips for my prototype, (instead of 27c040) since 512kilobytes is probably way overkill. But the pinout will be the same, so I could always drop one in later.
Assuming my prototype goes as planned I could be accepting preorders in as soon as 3 to 4 weeks, with production beginning when my summer vacation does.