The ribbon cable that connects from the header on the disk board to the serial cable or the serial port on the computer is of the same type as used on older i386 PCs, but there are two different types of 10-pin IDC to DB-9M ribbon cable in use, which look identical (the differences are hidden inside the DB-9 connector). Trust the "pee cee" industry to devise two totally different and incompatible cables and not provide a standard marking for them. Details on these cables have been committed to the OpenBSD installation document for the Armish.
So now I can see bootup messages, and interact with "RedBoot", the firmware boot loader these machines use.
RedBoot(tm) bootstrap and debug environment [ROM]
Red Hat certified release, version 1.93 - built 17:25:00, Feb 6 2007
Platform: THECUS N2100 (IOP80219)
Copyright (C) 2000, 2001, 2002, Red Hat, Inc.
RAM: 0x00000000-0x08000000, 0x0004b890-0x07fd1000 available, total: 128 MB
FLASH: 0xf0000000 - 0xf1000000, 128 blocks of 0x00020000 bytes each.
== Executing boot script in 3.000 seconds - enter ^C to abort
The next step was to install a hard drive as per the vendor documentation and OpenBSD as per those notes. These both went smoothly. The last step will be to make it automatically boot up when powered on...