Luckily, there's a workaround: plug the drives in to a USB hub, so they're not directly connected to the root hub. It seems that the BIOS will helpfully disregard them in that configuration, leading to normal POSTing. Right now I'm using a fairly old USB3 hub, and it's shaving 10 megs a second off my write performance and 30 megs a second off my read performance (via btrfs→openssh→ssh→osxfuse→sshfs→Blackmagic Disk Speed Test), but it's still a good 3x faster than USB 2.0, and a better hub might have better results.