Managed to find my old Xeltec SuperPro and some 27256 parts buried in the far corner of the basement, and so with the help of Jonathan Engdahl's KDJ11-D/S page, my CPU board now thinks it's part of an 11/53 instead of a Decserver 550. While I had the board out of the chassis, I changed the console serial port to 38k4, leaving the second serial port at 9k6 since that's what vtserver expects.

Telling the monitor to boot from SCSI disk flashes the drive activity LED, then tells me the media isn't bootable. That makes sense since I haven't put any suitable bits on the drive yet... and suggests I may be only another serial cable away from trying to install 2.11bsd.