Please forgive me if this is ignorant, but did NG somehow implement the ability to statically NAT the firewall's real IP? This was, of course, not doable in 4.1, which I understood to be more a limitation of the underlying operating system than the firewall itself. Please advise...
In any case, the goal can certainly be achieved by using a public IP other than the firewall's external address, then doing the route/arp/nat dance (or, client-side NAT in NG) to achieve the desired result.
A word of warning: do you really want Notes-type traffic passing in cleartext across the network? I'd submit that the answer is "no," especially if that network is the Internet (I'm not clear on that from the original post).
HTH
Dan Hitchcock