After making my post I realized I couldn’t use a diode because it’s AC current and the only way to switch AC current one way is with another relay. You can’t fit an additional relay AND a buck converter (to step down the 24VAC down to 5VDC) inside the hestiapi enclosure with everything else that’s in there (I tried).
I talked to a friend last week who has mocked up a new PCB with support for 6 or 7 wires and powers the pi off the common wire. This replaces the hestiapi Pi hat and mounts inside the enclosure the same way. Once I print it and test it works I’ll share the design but it may be at least a few weeks before I have a chance to do that. I know there are others that don’t want to wait for the crowdsupply campaign.