Broadcom calls this feature autofw (autoforwarding). Theoretically it does work as follows. Suppose, the external server answers to your queries at port 4444, In request to it this server should connect to your PC to port like 5555. In such a setup you should set 4444 as trigger port, in this case wl500g will forward all requests to port 5555 to your PC. Yes, it does have a timeout.
I've not checked the details yet, but full source code is provided.