Gents, after much thinking and tinkering, it seems the only way to wire the two boards together is with ... wire.
So, here's a new layout. You can see that the amp and PS are totally separate now. To use the onboard PS you just wire across the terminal blocks.
I've had to make adjustments to the PS in order to place output caps on the regs for stability when it is operated stand-alone.
Still talking with Jeff about how to get this out to you.