Hi Streambox. There may be a very ugly way of doing it but it requires your ISP hasn't got any sort of resrictions that prevent IP spoofing.
I recall once connecting 2 networks to a single server with 2 IP addresses
So 1 server had IP 123.123.123.123 (NetA) and at the same time had IP 234.234.234.234 (NetB). Each network connection required it's own gateway.
I had assumed that if you ping 1 IP, the 'pong' would come out FROM that network of which the ping was initially recieved but actually it goes out on the default gateway.
So outbound traffic would always go out of NetA even if the request went in through NetB. The outgoing packet did appear to come from NetB however.
So what you might be able to send out packets from a 'spoofed' address as long as you can correctly direct those request packets to the spoofing server from the public server.
It's kind of silly and ugly, but it might work.