Ah ha!

Many thanks to M L Riechers <mlr@rse.com> and his suggestion that the MESH
problems might be related to processor speed.

I replaced my XLR8 MachSpeed Carrier ZIF (300 MHz G3) with an Apple 120 MHz
604.  My SCSI busses now work!  Rather, MESH and an Adaptec 2940UW work,
esp still has problems (see my next post).

This reminded me of those old discussions about Retrospect having problems
on G3 upgraded PCI PowerMacs (i.e. original OF 1.0.5 7200 thru 9600
models).  And the XLR8 MachSpeed control panel which claims to fix this
problem before MacOS boots.

Then I looked in BootVars at NVRAMRC, and sure enough the XLR8 control
panel installed a nvram patch!

And, of course, running System Disk wipes out that NVRAMRC with its own
(but that doesn't seem to matter, since the NetBSD kernel still doesn't
work).

This would seem to imply that somewhere along the road, someone enabled
speculative processing on all G3 CPUs.  It would seem that the way to fix
these G3 upgraded PCI PowerMacs would be to disable speculative processing
on these models in the kernel.

Any chance someone could do this before 1.5 is cut?

Thanks,
  -- MW