Okay, that’s an educational rabbit hole that I couldn’t find on my own. Looks like the fact that Volumio runs hot all the time is a deliberate choice made by the Volumio devs:
My DAC runs hot all the time and the manufacturer says I should leave it turned on, so I can accept that this is a reasonable practice for the Pi. But you run Volumio in ondemand mode… seems like a simple enough hack. I’d love to hear what others are doing, though.