Basically, the situation is:
The external IP through which my connection goes is firewalled, and there is no way of port forwarding on that address.
Short of doing a full VPN to another machine, and routing through that and getting that to port-forward, couldn't the method I request work?
Surely all aMule would need to do is tell the server it connects to, the clients etc, that when they are to send it data, to send it to the external IP address that has a valid listening port, and to accept that it will send data from its own IP address that does not have a listening port?
Basically, listen on one IP address, and send on another.