Problem with 7.0 RC3 on headless Ultra 60

I'm new to this list, fairly new to Sun gear, and have some basic
familiarity with NetBSD (like I've installed it on a SPARCstation 5 and
Ultra 10 and played around a little).

We had a big clear-out at work, and there was a pile of Sun gear that I
rescued from scrapping, including three Ultra 60s that I'd like to get
working.  None of them have any video at all.  They are in various
states of disrepair.  NVRAM batteries are dead on all of them.

The one that seems to work the best completes its power-on diagnostics
and can boot the NetBSD 7.0 RC3 Sparc64 CD.  But it gets stuck at the
first terminal input, tail end of what comes out the serial port is:

warning: no /dev/console


You are using a serial console, we do not know your terminal emulation.
Please select one, typical values are:

        vt100
        ansi
        xterm

Terminal type (just hit ENTER for 'vt220'):

At this point it doesn't respond to typing either on the serial port or
the Sun keyboard.  The Sun keyboard works a bit at this point (Stop-A
breaks to the debugger, and it can power the system off).

The serial port works for console output, but not for input; I have to
type on the Sun keyboard to interact with the firmware.

I feel rather ignorant about these machines, can anyone with more
experience offer me a little advice?