Tonight I shut the box down, plugged in the controller, plugged in the tape drive, and turned the whole mess on. Linux spotted the SCSI controller and offered to configure it for me. That was mighty nice of it.
I then stuck a tape in the drive and tried copying files to it. It worked!
My expectation was that I would spend an hour or two rebooting and troubleshooting, but instead the whole operation took about ten minutes.