But...  While searching for a solution, I managed to lay my hands on
   an SBUS Sun 8-port serial + parallel card, which identifies itself to
   the kernel as
   
     SUNW,bpp    for the parallel, and
     SUNW,spif   for the serial
   
   It seems there isn't a NetBSD driver for this (although there is an OpenBSD
   one apparently.)

i have one of these cards.  i have the beginnings of a driver for it.  i
was not aware of support for the spif in openbsd.  are you sure it isn't
just the bpp support?
   
   So, more questions <grin>.  Does anyone have a driver 'under construction'
   for one of these beasties?

the bpp driver in NetBSD is currently in the process of being fixed.. i
am working with der mouse to get his changes to make it at least work
into the master system, and it should be done soon.  i can send you a
patch for this, if you want to try the parallel port.  it is send-only,
not bi-directional (as hardware supports.)
   
   If not, I'd be happy to try writing one myself; I've never written a
   driver for NetBSD or SBUS before, but I did used to do embedded software
   engineering so it shouldn't be totally beyond me <grin>...

i can provide pointers (or copies if the pointers are stale) to most of
the docs needed for the stif card.  there are some points i was never
able to work out about the sbus interface which is why i stopped working
on the driver.  i can send you my code as-is separately if you wish.
   
   My other question is, therefore...  Where is a good place to start
   looking for information on how to write a NetBSD d/independent SBUS
   driver...?  Is the OpenBSD code worth looking at, or is the way they
   do things totally different?

have a look in sys/dev/sbus.  in here are almost all the sbus drivers
in NetBSD.. these are shared between the sparc & sparc64 ports.  the
actual sbus drivers themselves are implemented in
sys/arch/sparc{,64}/dev/sbus.c.  i have not looked at the openbsd sbus
code for several years, so i can not say if they have kept up with our
changes or not.