yeah I remember that because we talked about it and I got F8 dumping working. I tried setting the MM78's test pin to different voltages but it only seems to put it in or out of test mode. I tried 0 to -10V (normal range) and then all the way to -20V via a resistor.. no change. the IOs are good to -30V so claims the sheet so this was safe.
I tried poking data into the other pins but didn't get any different behaviour; this chip has a serial port on it too so I applied a clock to the clock pin for it and could shift data through the chip's serial port but I don't think this was doing anything useful internally.
The MM76 chip dieshot would probably help a bit, since this 78 is very similar I think, so both will most likely dump the same way.