Here's the update...
New bank angle sensor arrived from Stadium Yamaha. While I was waiting for delivery, I re-tested the voltage on the original part and discovered that I had previously been testing it incorrectly. The part was fine. So I moved on to the next likely culprit, the ignition control unit. I swung by a local scooter shop, Kymco dealer, to see if they could test it. No dice. They had no diagnostics for it. So I just ordered a new unit, clipped it in, and bingo.
Since I had all the panels off, I picked up some low voltage LED strips for $18 at an auto parts shop, wired them into the headlight and mounted them on the undercarriage (2 - 28" adhesive strips). They look pretty cool and improve side-view visibility at night. I may add a third strip to bump it up. It's easy to get carried away on this sort of thing.
Thanks for the help and suggestions on this board. I only missed one month and spent $160. Not too bad. And thanks to Stadium Yamaha for having the gumption to put their parts catalog online. Very convenient.