Well, thanks to a custom kernel from Allen Briggs based on information from
Michael Zucca I have what seems to be a fully working MacBSD/Quadra 840AV.
Thanks a lot guys!

I can't seem to find the machine status web page at the moment, but from
memory the relevant items are:

Onboard Ethernet--not supported (uses the MACE chip)

NuBus Ehternet--Now Supported!!!

Onboard Video--partially supported (Michael Zucca seems to have doubts.  I
have no problems now at 640 X 480 @ 1-bit, but it bombed at the video setup
before I zapped PRAM.  I have minimal need for video on this machine and
probably will not experiment much unless he asks me.  I trust the bit about
zapping PRAM is in the FAQ somewhere.)

NuBus Video--should now work as well as any other mac AFAIK.

SCSI seems to work fine, but I'm told DMA is not yet supported.

Caveats:

Booting seems "fragile."  I can't seem to boot into *BSD if I have changed
configurations that affect PRAM, like turning Appletalk on, or if I use
TCP/IP, like using Fetch.  Cold boots seem stable enough, and running the
MacBSD utils like the installer don't seem to cause problems.  I'm running
7.6.1 with Open Transport 1.1.2 and Appleshare 3.7.

This machine has not yet been stress-tested, just set up.  I still need to
get a fan for the two drives sharing the lower internal bay before I do
very much with it.

Doing a shutdown -h now does not turn power off, but it does print a
message that you can do it by hand.

Signature failed Preliminary Design Review.
Feasibility of a new signature is currently being evaluated.
h.b.hotz@jpl.nasa.gov, or hbhotz@oxy.edu