I took the changes Jonathan told me to apply to 
/sys/arch/pmax/pmax/conf.c, built a new kernel, tried the Xmfbpmax binary 
from Arne and... got a few sub-stamp-sized 'icons' of the expected X 
display onto my screen. I then tried, as was pointed out several times in 
this list, the Xcfbpmax, which seems to really work fine... really a good 
thing! Thanks again to all who have helped me get that far.

The annoying feature of this solution is that the server gets recognized 
as a color display (xrdb -symbols reports, among other things,
-DPLANES=8 -DBITS_PER_RGB=8 -DCLASS=PseudoColor -DCOLOR). Hence, all 
#ifdef COLOR etc. in the .Xresources or .Xdefaults aren't really useful 
any more. Is there any way to tell the server to report that it is a 
monochrome one (I would even consider re-compiling it from the patched 
MIT sources if there was no other way)?

Helge