Ironically enough, I've found myself in a position where I need to build
a resi-grade gateway device for my own home.

To get straight to the point, my only imminent requirement is to be able
 to direct all ingress traffic on the LAN side of the box to an external
HTTP proxy server on a remote network. Essentially, anything ingress to
80/443 is redirected to x.x.x.x:8080 outside of the WAN interface. Is
this possible?

The WAN interface can be configured as a PPPoE client, or simply routed.
The WAN interface will be connected to a device which will terminate an
ADSL connection.

If this functionality is possible, it will provide me with the ability
to also help test IPv6 functionality of m0n0wall, tunnelled to within my
own (employers) infrastructure.

Steve