MITA sent me another set of diagrams for the DT board (would of been nice if that included schematics or a parts listing but this was still extremely helpful) and I learned that all of our cooking components are connecting on the bus on pins 9 through 16.
These are part of whatever the MMA signalling is. I was suspecting that these lines are being shorted to ground, but it's weird that nothing else using those pins was not complaining just as hard and I didn't see anything else causing the failure. I pulled the logic probe out and began to poke around with and without the IV/90 cabinet powered on and found everything was dead in state. I brought out a scope and again found the majority of the bus was doing absolutely nothing other than going low once the expansion cabinet was turned on. I checked for the four clock phases which are accessible from four headers at the top of the memory boards (and on the backplane) and phase 1 and 3 were missing, phase 4 would pull itself down to a negative voltage and do nothing and phase 2 was generating something that looked more akin to a shark fin. So yeah, missing clock phases would totally explain why everything is completely unresponsive.
Checked to see if it was the front panel in MANUAL and it was expecting single-stepping an no.. Checked to see if the clock was being generated by the IV/90 and no the waveforms were the same even with the expansion turned off.
I started pulling boards until the phase I DID have went away. Finally I was able to conclude that if the DT board was pulled I'd lose it, but if I instead pulled the IV 70 Driver Receiver Board that too would take the clock phase away. Furthermore the resistor pack and transistors stopped overheating and the last six bits of the front panel would get stuck all-on, so this was a clue. I didn't know what it meant, but this was progress.
It is the least complicated board in the system but I'm not sure what it's doing. We can clearly see pins 9 through 16 here as they run into a 7414, a 72365 and an N8242. I still do not know what this board is doing and it doesn't actually use phases 1 or 3 but it does use the other two phases we saw doing *something*. Nothing that I can tell on this board is hard shorted and being a 5v-only board I can power it on the bench. I do however see a few warmer-than-normal IC's, such as that one 7414. I wish my Logic Comparator didn't suck.