I think that if Palm IIIc has the same interface as Palm V, it's RS232...
so : it's only a IO port, but the system cannot manage by default an external memory for its own...
as far as i know : I'd never find such schematic....
But if you want to develop your own board to inferface with a software of your own the flash : you'll simply need the flash, a MCU of your choici which a capable of RS232 communication, and a MAX232 : to level-up your MCU signal to the palm RS232 signal...
after that : you'll need to develop the MCU firmware as a bootloader (for read and write access)
and the appropiate application on palm. Then : only your application could manage the flash : but not the PalmOS !!
bye