I had come across the utility in my Googling, but I'm pretty sure it doesn't work for USB drives. I got the following result:
# hdparm -S 0 /dev/sda
setting standby to 0 (off)
HDIO_DRIVE_CMD(setidle1) failed: Input/output error
For the moment, I have a makeshift solution with a script that creates/deletes an empty file on the drive every 200 seconds while it's mounted, but that seems awfully hackish.