Dave, I have a pretty good understanding of how routers work. If you want to get to any device from the internet, it needs to be at a publicly routable static address, regardless of the port number. It can not be hidden behind a NAT segment.
When someone wants to reach that server, however, the router THEY are behind will block OUTGOING requests on ports not specifically opened. Since the vast majority of those requests are on port 80, it's almost universally open. If you put your server on another port, it may be difficult to reach from many locations, unless you create a specific application and use UPNP or something.
I suppose what you are doing is having the fixed IP source router route a specific port request to a specific internal address and port - clever idea. But if you are doing this, and no other server is hosting anything on this address, why isn't port 80 as good as any other? Or are hosting companies blocking this to prevent people hosting from their homes?
I really am just curious - most of the things I do in this space are on private networks, and I'd be looking to serve internally only from this sort of device, with no need to access it from beyond the corporate firewall.