Any suggestions on backing up and restoring whiteouts?

I've got a bunch of whiteouts in the top layer of my union-mounted
NetBSD source tree.  dump(8) ignores them as far as I can tell (they're
indoe #1), and restore(8) has no way to create them (There is an
undelete system call, but no mkwhiteout() syscall--they're only created
by a union mount).

Any suggestions on how to automatically back them up and be able to
restore them?

==John