Well, the first of two HPs is now online and running NetBSD, thanks to the help of Mark. [1] It seems I needed to have bootparamd running, as well as NFS. Also, the documentation neglected to mention that you might, just might, need to hook a terminal up to the machine.
Lo, there was SYS_INST, running not on the console (the wonderful 21" monitor) but on the serial port. Sigh. After that it was a painfully long process of transfering the installation program via NFS (although that may be due to the logging we were doing) then the actual install process (via FTP, much faster).
Slackware Linux was a breeze to install compared to this. The whole disk labeling is confusing—I find the PC scheme much easier to deal with (although Mark finds the PC scheme too braindead and likes the added complexity that disk labels bring. Go figure). We still haven't figured out how to get the system to boot off the drive, but it is running.
More on the exact steps later …