Okay, non-issue. At the thermostat, with my new thermostat installed and connected, the rectified voltage measures 34.8VDC. That's with the three extra diodes inline to provide a small drop. Looks like they might not even have been necessary.

(previous AC measurements were with the original thermostat in place, not my new one).

So plenty of voltage headroom at the input to the buck. Peachy!
The photo shows it in place, sans cover. The white thing above it is the controller for the HRV air exchanger, which I hope to eliminate in favour of a couple of the spare yellow relays in the new thermostat below. That's the next project.

Except somehow I've managed to fry three (3!!) of my FTDI USB/serial breakout boards.. must have a wire crossed somewhere there. So I cannot actually program anything new until those get replaced -- ordered the raw chips and a supply of ChipQwik from Digikey so that I can just replace the blown chips and reuse the existing Sparkfun breakout boards afterward.