My ISP assigns me a public static IP address but for my gateway
machine's WAN connection they give me a static RFC1918 address. They
must do some static transformation of my packets at their end, mapping
between the RFC1918 address and the public address.

This messes up my ability to use connection tracking to support
protocols that cross my gateway's IP masquerading: it's putting the
wrong address into the protocols when it rewrites the content.

Is it possible to have it write the public IP address into them somehow?
I can imagine I can do it if I give its WAN interface the public
address, then have another not-connection-tracking 'outer gateway'
between the gateway and my ISP that does the reverse of the
transformation they do.

But, can I achieve the same effect without needing another machine?
(Or another ISP. (-:)

-- Mark