I just started to try the union fs so I may be missing the obvious,
but it seems that mount_union only works read-only, which rather
defeats the purpose:

wheel { /tmp } % ls foo bar
bar:
ack

foo:
blah    xyzzy
wheel { /tmp } % mount -t union foo bar
union: Operation not supported
wheel { /tmp } % mount -r -t union foo bar
wheel { /tmp } % ls foo bar
bar:
ack     blah    xyzzy

foo:
blah    xyzzy
wheel { /tmp } % 

   I can do a null mount between another directory and the mount
point to get write access, but that seems rather convoluted. Also,
I noticed that if you leave a trailing / on the mount directory
for a null fs mount then you can't umount it later because umount
thinks it isn't mounted. Can anyone else recreate or explain this
behavior? Thanks.

                                      Gary D. Duzan
                         Humble Practitioner of the Computer Arts