Alrighty, sorry for the delay - the diagnosing and repair took all afternoon on Saturday, but in the end, I've got it fixed and the fans are running as they should be now.
So to recap: Radiator Fans were not kicking on and while doing a lot of low speed manoevering during a training class, the bike overheated. Coolant levels normal (though I did take the opportunity to top up the radiator which was not quite completely full when the bike was checked on the sidestand).
What I was able to troubleshoot on my own:
1) Shorting out the plug to the temp sensor did not turn the fans on
2) pulled the tank to check the next connector upstream - connector was tightly connected, no signs of corrosion.
Somewhat at a loss of what to do, I hopped on the phone, hoping Gerry could point me in a direction - he did better than that, he helped me diagnose over the phone and gave me a suggestion for repair.
So here's what we did:
1) Checked voltage at the plug to the temp sensor and found it to be nonexistant (when the ignition key was turned to on but bike not running), and seriously low while the bike was running
2) Checked voltage at the next connector upstream, and also found it to be low
3) checked voltage at the fuse box for the relay in question and found power to be fine
4) checked voltage at the black triangular connector near the solenoid (the connector is the one next to the cylindrical connector with the purple ends), found power to be fine (all pins hot)
This meant that there was a wire that had gone flaky between that black connector and the diode. Possibly where the the wiring harness gets squished next to a bolt on the frame closer to the front of the bike.
The options presented to me were 1) get a replacement wiring harness and run that 2) button it back up and take it to a shop or 3) run a parallel wire - I chose option 3 - and I completed it with splice connectors/plier taps. A solder join would definitely have been more secure. Since I had to cut open the sheathing around the wire bundle that goes to the diode a fair bit, I secured the whole lot after the repair (new red wire) including the splice connector with self fusing tape. The other connector I did not do the same to, because it's pretty well shoehorned in place and hopefully will not work itself loose.
During the course of this adventure I learned all about the judicious use of paper clips for back probing and jumpering, how to use a wire tap/splice connector and I learned that I need to learn how to do a solder join. Also learned that the colors noted on the wiring diagram may not be consistent with what is in the bike.
Before putting everything back, I checked the voltages at all the connectors that I touched to ensure that i had power where I should, and at the correct value. After everything was buttoned back up, I made sure the temperature sensor was also working by leaving the bike idling to get the engine temp up to 3 bars, at which point the fans happily kicked on.
Now that I'm done with the repair and playing the what-if game in my head, I'm wondering if I was too eager to do the repair myself, because if something goes wonky down the line I'm not sure I'm going to be able to get those splice connectors off to attempt a soldered repair.
Ah well, and since it's always "photos or it didn't happen" here you are:
oops, one more thing - I had a tricky time getting the tank back on, probably because i had been moving things all around, and the fuel lines generally didn't want to go back nicely under the tank. I did ask if there was a trick or tip for disconnecting the tank and Gerry suggested disconnecting the fuel line below the fuel filter - rather than the 2 lines directly at the tank.