Tough one; I couldn't find any data on those CPUs. I guess one could figure out at least some of the address and data lines by tracing them from the on-mainboard ROM and RAM chips (standardized pinout) to the cartridge slots. That would at least give us 8 or 16K of memory space. The rest is then likely to require lucky guessing or even logic analyzer while the device is running.
You seem to understand a lot more about this process than I do. By any chance is this a task that you would consider?
The only vague technical info I could find was on Wikipedia:
CPU: UMC UA6588F (earlier revision); NCR 81489, 8 bits (BIT WS39323F) in a QFP-100 shell (later revision)
ROM: 2 KB (UM6116M-2L CMOS static RAM, pin compatible with ROM/EPROM chips)
RAM: 16 KB (2 x CXK5864M-15L chips) of static RAM
Media: ROM card, very similar to Hu-Card (PC-Engine), My Card (Sega SG-1000) and Sega Card (Sega Master System) 19x2 pins