Why don't you just try to replace the SASI-to-MFM-plus-HD part
with a SCSI disk, and modify the kernel appropriately 
(if even necessary)?

I've done a 6502 SCSI interface based on an old SASI interface
published by some magazin, and used it up to a 4GByte 
SCSI-II Ultra-wide (with 16-to-8 bit adapter) SCSI drive
in async mode.

See
http://www.6502.org/users/andre/csa/scsi/index.html
and 
http://www.6502.org/users/andre/hwinfo/scsi.html

Although, IIRC last time I looked at the D90x0 schematics,
I actually didn't find the SASI interface, but I might have missed it.

André

> I think the info about SASI was largely in Shugart docs, and probably
> mostly with the S-100 crowd.  By the time disk were hitting the home
> market in quantity, SASI was replaced by SCSI.
> 
> If you know anything about the earliest SCSI implementations
> (especially how drives and controllers handled the lack of an IDENTIFY
> packet to query the drive geometry), SASI will look very familiar.
> It's probably possible to untangle the code on the DOS board by using
> an ACB4000 programmers manual as a guide to deciphering the packets
> sent.  Knowing what packets the "middle board" supports would be very
> helpful, but one could make educated guesses based on other SASI and
> early SCSI "bridge" boards.

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