Unionfs etc Re: Packaging

[Rahul Siddharthan <rsidd@xxxxxxxxx>]

# mkdir /tmp/a
# mount_union /tmp/a /usr/local

works fine and if I create a file in /usr/local it gets "really"
created in /tmp and is only visible there after umounting /usr/local
-- just what I wanted.  However,

# mount_union /tmp/a /
says 
mount_union: /tmp/a (/usr/tmp/a) and / (/) are not distinct paths

Is there a way around this?  I ask because I'd rather not assume
anything about PREFIX, and I'd like this to be usable for
badly-behaved installers that put stuff into other places like /etc,
requiring only that (a) it doesn't clobber anything pre-existing, and
(b) I keep track of everything installed so that it can be removed,
in +CONTENTS, pending your VFS stuff.

Another question: the FreeBSD porters' handbook requires you to create
a directory tree first in your temporary install directory when
building pkg-plist, and add any newly created directories to pkg-plist
with a @dirrm -- is this really necessary?  It seems to me that we can
just make pkg_delete (or its successor) remove any empty directories
it finds after deleting files, including /usr/local/share or other
"standard" places -- there's no reason to have these directories if
there's nothing in them.

Rahul