<So, is this a technically feasible abuse of SCSI?  And if so, how hard

Yes, it was done in the mid 80s using z80s (amproLB+).  The idea of scsi 
mutimastering is not new and offers a fast though physically short bus. 
for the 3100 class machines this could offer a possible replacement for
mop/eithernet.

<would it be to make driver for the SCSI devices that made them look like
<network interfaces?  AND if we could intersperse a SCSI disk drive or tw
<in the chain, we might have a faster alternative to ethernet for shared
<disk storage.

All doable.

Allison