I didn't get well your objective, but anyway bbsm always work with nat. Its internal LAN NIC provides non-valid (private) IP Address to bbsm users, and which are only known by bbsm.
The bbsm "external" LAN NIC may have an interface with a single valid IP Address which is shared among all bbsm authenticated users. FYI, this external interface may also work with a private IP address, behind another nat network.
Hope it helps
Leonidas