Okay, finally got in the tether that I ordered (one of the several things missing when I got it), so it was off to the races to install the board tonight. Knew the motor worked thanks to Adriatic's 9v battery suggestion above, and the rest was fairly simple. 4 wires off the tether, 2 for the program/run switch, hook on the antenna (which will eventually present a bigger issue, as the entire tender shell is die cast, but that's a problem for another day), biggest issue was finding a connector that'd fit to wire the tender coupler to the board.
Powered up, twisted the knob on the Cab 1, and off she went. In reverse, of course, but that was a simple enough fix. Salvage operation (phase 1) complete! As I mentioned, permanently mounting the antenna will be a challenge. It's probably either isolate the body (anyone know a good source for nylon screws?), or dremel off the cast coal load and replace it with an actual one (with the antenna mounted below).
The (original MTH) electric coupler on the back of the tender, does polarity matter (both wires from it were black)? And can in only be triggered in TMCC mode? Thanks for all the help and suggestions so far, guys!