Excerpts from netbsd: 23-Oct-97 How to unmount a double mou..
PxsoQg==?=@kw.netlaputa. (879*)

> Assume /usr/local is on a separate FFS partition.
> [...]
> > # mkdir /tmp/local
> > # mount -t union /tmp/local /usr/local

I have a similar setup running:
zelnfs:/netbsd-checkout/src on /usr/src type nfs (read-only)
/dev/wd0a on /altroot type ffs (local)
<above>:/altroot/src on /usr/src type union

> > # umount -g /usr/local
> > /dev/sd0g: unmount from /usr/local
> > umount: /usr/local: Device busy

I get a similar bogus printout, but the right thing happens:
(your -g is obviously a typo)
zelz26# umount -v /usr/src
/netbsd-checkout/src: unmount from /usr/src
zelz26# mount
[...]
zelnfs:/netbsd-checkout/src on /usr/src type nfs (read-only)
/dev/wd0a on /altroot type ffs (local)

So perhaps is your filesystem really busy?
There are indeed some problems with the mount utils
when it comes to real filesystem stacking (they do
usually a statfs() and believe what they get...), but
I would double check the other possibilities in your case.

best regards
Matthias