Maybe more for idle fun, but also to aid in diagnosis, I am working on an Arduino shield for the Mega 2560 that connects to the expansion port on the ColecoVision, in order to diagnose as many hardware problems as it practically can. I'm only in the beginning stages at the moment. First check is clock functionality (easy enough), then that the bus behaves like it should (check that the processor disengages when told, and that none of the bus lines are shorted). Planned additions will be ROM checksum, RAM and VRAM checks, cartridge ROM check (though this would require a 32K cartridge to fully check all access lines), sound and VDC checks, and possibly even checking the RF and video switcher directly.
The shield board is not technically advanced--basically it just maps most of the expansion port pins to Arduino pins, and the diagnostic software will do all the testing itself.
I did add a rudimentary DAC that I mostly cribbed from the Atari for video output, though it will only truly be useful for black/white images at best. Still, if I enabled the external video and sent a "composite" signal through it, the RF modulator should be able to make it appear on the screen.
Anyway, just ordered a prototype PCB for this. We'll see how long it takes to arrive.
If interested, I've added the board design to my CV schematics under the Diag sub-project.