I sup into a "base tree" which I have union mounted underneath my
working tree.  (I haven't enough space for a CVS repository *and* a
working area).

You will want patches to the union filesystem (I sent them in via
send-pr; an updated version should arrive via the union fs authors
sometime soon).

from my fstab:

/dev/sd0f /u1 ufs rw 1 2
/dev/wd0e /u2 ufs rw 1 2
fdesc /dev fdesc rw,union 1 2
/u2 /u1/NetBSD-current union rw,noauto,-b 1 2

(noauto since "below" union mounts create the shadow dirs using the
mounting UID.  I run 'mount /u1/NetBSD-current' by hand under my UID)

I then sup into a tree at /u2, and look at it through the union at
/u1/NetBSD-current.

==John