Is it possible to export via nfs the mount point for a union mount?

I have a server on which I am mounting the distributed 1.2 source
beneath (i.e., via mount_union -b /usr/src-dist /usr/src) another
partition in which local changes occur.  The lower partition
(/usr/src-dist) is exportable fine.  The upper partition (/usr/src),
however, is only exportable if not union mounted.  Note that both
/usr/src and /usr/src-dist are mount points for disk partitions.  How
can I get both partitions 1) union mounted and 2) exported?

Thanks for your help.

Cheers,
Brook