I have a Quadra 950, and about a year ago I played around with trying to
get MRG_ADB working...unfortunately, with no success.  I was able to deduce
the data structures the ROMs use, and call the initialization code --
including loading what I presume is the 6502 code into the IOPs -- but I
couldn't figure out how the IOPs pass back interrupts.

I've got patches against an approximately year-old snapshot of the kernel,
and a whole bunch of assembler dumps of my ROMs, if anyone wants to try
playing around with them.

The serial port, or at least the serial console, doesn't work, despite the
supposed compatibility mode of the IOP serial controller.  My guess is that
either the compatibility mode depends on support from MacOS, or somewhere in
the serial-port initialization we're clearing the compatibility flag.

The external SCSI bus seems to semi-work -- some code I tested for Allan
correctly recognized an old external HD I had.  Unfortunately, that drive
has since died, and the code *doesn't* correctly recognize my external
CD-ROM drive (it dies complaining about parity errors, or something -- I
don't really remember).  It's not clear to me whether this indicates that
the external SCSI bus isn't fully working yet, or there's something wrong
with that drive (it works under MacOS though). 

-- 
Jonathan Lennox
lennox@cs.columbia.edu