(We are currently running the 400's diskless under HP-UX 8.0, so I
   figure it should be possible if NetBSD has the necessary stuff (It
   looks to me like it has all the necessary pieces, but I'm not 100%
   certain...))

There's one critical piece missing: a boot program to load the kernel
over the net.  John Brezak had an unfinished one a while back, but I
don't know whether or not it's ever been completed.