Should I crack the unit open and see if I can get a serial console?

You'll need a little inverter board to do this, which plugs into a 4-pin header on the main board. Hugo might be able to help you out with the circuit diagram.

We've never tried it with an OpenBSD NFS server, so there could be something wrong there. ISTR that we had problems with the read packet size causing really slow bootups on other NFS servers (particularly ours).

However, if it's doing some NFS and then starting again, it sounds like it's crashing and rebooting. A serial port is probably the only thing to try here.

Generally, if the service locator request (for the HTTP server) is unsuccessful, you're left at the bootup screen with a (cryptic) failure message.

What does the screen say (exactly) -- the second kernel displays a different screen, so we can tell if it's successfully loading that.

Oh, something else occurs. Do you have the necessary layout files installed? I think it reboots if it can't find one.