After studying the FlexBOSS user manual's RSD section, and then comparing it with the tips from their 18KPV manual:
I have come to the conclusion that EG4 just made a mistake in the GridBOSS manual. It really is as simple as breaking the 12V from the SW input to cause RSD. This implies we should connect the +12V output from either BOSS unit to a NC contact on the mushroom, then running *2* wires from the other NC contact to the 2 "SW" inputs.
Now, there are two ways to cause the CCA to go into RSD: (a) remove its 24V power, or (b) short the two AUX pins. If you want logging to continue working, use option (b). That's what I plan on doing with the NO contacts of the mushroom.
So yes, that means we'll need the NO and the NC contacts of a single mushroom button (good thing they generally provide both). The NC is used for the BOSS units, and the NO is used for the CCA's AUX pins.
BTW, the FlexBOSS has a 24V output on the RSD wire block that can used to keep power on the CCA at all times. I plan to use that rather than an external 24V wall wart.