So after seeing this I was inspired to try making my own "magic spray" using the mech board from the El DohNaDo project I recently acquired. Practically nothing to lose there, so I considered it an experiment.
Basically I sprayed LA Totally Awesome cleaner, with comparative spritz or two of Simple Green. That seemed OK, but Purple Power really seemed more effective at knocking off the worst of the scuzz. I used an old toothbrush to scrub the relay switches, and a water spray bottle to rinse everything. I didn't scrub the wires.
I left everything attached to the board because the iffy condition of this panel had me worried about breaking or creating flaky connections. If the board goes bad, eh... it's just plywood and I'll deal with the gamble.
I used my air compressor to knock off the excess water and aid the drying process. I discovered that saturated fish paper in the score motor switch stacks was pretty fragile, so use caution.
I didn't plan to do this scientifically and only had a few moments to rush into it, so unfortunately I didn't take a "before" picture to really focus on just how nasty this board truly was.... trust me when I say everything had a sepia tone and felt rough and nasty to the touch.
Best I can do is find a "decent" before photo, and stage a retake with the after board. The photos don't really do it justice: the colors pop, the metal is shiny, everything feels clean to the touch, and the switch leaves - which are not really visible in either photo - went from "nicotine" to "silver".
Still a lot of work to do before this is ready to test in a game, but for a half-hour's worth of work, I'd do it again.