You could imagine something uPnP-like (or maybe just uPnP - I'm not too sure about how that protocol works) to dynamically create <remote_addr> -> <my_box> mapp

You could imagine something uPnP-like (or maybe just uPnP - I'm not too sure about how that protocol works) to dynamically create <remote_addr> -> <my_box> mappings; otherwise, just drop the packets. (Software already works behind bad NAT routers - dealing with this particular atrocity is nothing new.)