What I think you should look into is a simple firewall or proxy service that will
let you do adress translation. (NAT)

You could try some of the simple Linux firewalls on floppybased bootdiscs.
Not sure if this will give you NAT functionality.

The easiest, not necesary the cheapest way, is to buy a xDSL router/firewall as they often give you NAT functionality.

Not exactly what you was hoping for I know, but maybe a step in the right direction.

TommyE