Putting the logic analyzer on it was all over the place, to the point where I couldn't make sense of what I was seeing. But I also determined that my console's power switch is wearing out... so I need to make a repair there to continue anyway.
One thing that has crossed my mind is that with the new compiler, the code may be running too fast. I check the MDIR and MODE pins immediately after detecting GSEL -- maybe that's too fast? When I get the logic analyzer properly on the bus I will recheck that theory.
This shouldn't even be an issue, but I am leaning back towards the ROM-based cart.... I just want this part to work. ;) But in the meantime, I relaid-out the cart to use the ATTINY861 instead - it's a 20 pin part instead of 14. So it doesn't look as much like a GROM, but it has enough pins to bring out the ISP header. I also positioned it so, in theory, the ZIF socket will fit on the board when in the console. I haven't ordered this board or the new parts yet though...
I probably should test programming one of the 128MB flash chips as well -- if my new programmer can pull that off, maybe I should stop wasting my time with the GROM emulation. (But darn it, this part was supposed to "just work". ;) )