unfortunately we don't have a dump of the protected mask ROM, but it seems to be somewhat similar, and the 6502 puts *every* address it accesses on the bus, so you can see it accessing special function registers and the stack, allowing you to infer quite a bit about what is going on.
here's where things get really interesting. i've pulled in the imagery of the early ST-251 board and overlaid my Kicad traces from the newer revision.
it's almost like a visual diff--now i can see exactly what changes were made between board versions, which means i can back out a schematic as well!