I found a different solution.
New ASUS Vivobook. Upgraded to 8.1 Pro and enabled BitLocker. Rebooted upon command. Got stuck in the auto-repair loop until it finally refused to even get past the "Preparing Auto Repair" message.
Turned off computer. While powering back on, I tapped F2 repeatedly until I got into the BIOS. Disabled Fast Boot and exited, saving changes.
Rebooted to BitLocker prompt. Entered encryption password and booted normally.
Will leave FastBoot off until encryption completes; then I'll test to see whether it needs to be left disabled permanently.