It might be a jumper on the board. Mine (Q770G4) boots on power, if I can organise some downtime with the family I'll take a look at it (set it up ages ago so can't remember).
Edit: CAB approval was easier than I expected! Mine is in the BIOS, under Chipset > PCH-IO Configuration, set State After G3 to Power On.
I wonder if this kind of thing might make conversions into older cars more viable. If the body and the million computers etc will cost heaps to fix in a newish EV, that might mean an increased supply of electric drivetrains at the wreckers. They're not exactly a drop-in proposition but having the parts available has to be a good start.