Correct me if I'm wrong...

Under the settings menu in the NetBSD Booter select console on serial port
and set the baud, handshake, etc as you wish.  Connect a serial ccable
from the port which you selected in the booter to the modem port on your
other macintosh.  Start up a terminal application (AppleWorks has one,
there are tons of free ones) on the other macintosh.  Make sure that the
settings (baud etc) are the same.  Connect from the terminal app.  Select
"Boot Now" from the booter.

That should work.

Brendan