Well, it turns out that the solution to my "getting grub installed"
problem was actually quite simple.

On my system wd5 doesn't exist, so I made a device node as rwd5d with
the same major/minor numbers as my array (copied from /dev/rld0d) ...
and this was a device grub scanned, without issue...

I filled /grub with the various files it needs, and then:

grub> root (hd0,a)
 Filesystem type is ffs, partition type 0xa9

grub> setup --stage2=3D/grub/stage2 --prefix=3D/grub (hd0)

That was all it took.