Here’s a tangent discussion from a while ago that I enjoyed, on bootstrapping PoE (Power over Ethernet) from UEFI: https://news.ycombinator.com/item?id=44111609

Looking back, funnily the top comment drew a parallel to negotiating USB-PD in u-boot, aka the bootloader. I suppose this wouldn’t have worked for your case though, since your device couldn’t boot at all on 5V.