Maybe I'm misunderstanding what you're trying to do?
In my case I just recently had a site where the customer was using an ip address to direct traffic at a database/application server.
The client software was configured the same for users both in the building on the local lan and outside the building at remote sites.
The Thompson router supplied by the ISP simply redirected the requests for the ip/port combination to the internal machine.
Even the internal machine (server) was being used as a client, and it's client software would send requests to the router which would then come back at it self. (Yes I know how dumb that sounds, but it worked and they weren't about to pay anyone to set up a more technically better solution).
However, I can see what the 750 can do, so I think I'll go off and have a play and just nut it out.