After much sifting through other topics, I stumbled across this nugget of information:
Since I have also decoded the schematic enough, I know that backpowering through USB is, aside from a 2A fuse, just the same as powering through the VCC5V pins on the header, so that answers my original question, I guess. But of course that leads to new questions about how it behaves when so powered:
“Unable to reset power via regulator”? What’s that mean in plain words?
I have some inkling about the SoC watchdog, but from what I’ve read it causes other annoyances if enabled, no?