That's a very kind offer, and thank you. However, I'm delighted to announce that I won't have to take you up on it. I nailed this sucka.

Turned out there were two problems, closely related: First, the fuse supplying this clutch control circuit was blown - I found it thanks to the diagram supplied by HeyYou, which actually synced with the (incomplete) Haynes drawing. Second, one of the three pins on that clutch cycling switch had a busted connection - the wire strands were all broken, but it was hanging on by the insulation crimped to the pin.

 

It works now? Way cool!