@Reg I can patch the Chr routine while it's running and get it to boot and play. Alpha display (second half) is a bit broken in this video, but I jiggled around some settings and then it looks like it may display correctly. Does seem to expect standard-ish buttons/ and coin button mappings:
Trouble is that to do this as a ROM patch, I'll need to also figure out potentially a bypass for the checksum routine. Once I've done this patch on the fly, attempting to reset the machine results in no boot. This is almost certainly due to my patched bytes meaning the ROM checksum is out. Probably not quick...
To boot, you need to add a breakpoint on 0800 (standard Chr address), once it hits that during boot sequence, you need to set bytes d1df and d1e1 to zero. Then resume past the breakpoint and the machine will boot.
Tagging you @andrew96 as I know you're interested in this sort of thing If someone has time to figure out a checksum bypass patch, then the hack above can be made in the ROM image, and then you should have a booting ROM. Whether that can be made into a playable game in MFME, I've no idea - I can't tell if it's working properly or not from that play in the video above!
After changing some settings, one of these meant that the Alpha looked to be working properly: