Regarding the PlayChoice-10, can that extra 8KB at the end of the ROM file be put to any use? Supposedly it contains Z80 code and hints that appeared on a second monitor. Is there enough data in the ROM file to emulate that second screen?
If my understanding of nocash's description is correct, yes, you should be able to HLE the second screen without needing the PC10's BIOS.
You might even be able to get away without needing the associated bitmap font ROMs and just map the glyphs it to unicode points.