A generic-pc .img based on 1.2b7 that features TransProxy v1.5 and the 
corresponding .diff against rootfs-1.2b7.tar.gz are available on 
http://crosscom.ch/m0n0wall/

So far I have tested only the following scenario, as briefly described 
in 
http://m0n0.ch/wall/list/?action=show_msg&actionargs[]=159&actionargs[]=38

- PC with one single NIC (LAN);
- Captive Portal enabled on LAN with (kind of) AUP, actually to inform 
clients that they should kindly reconfigure their PCs to use a HTTP 
proxy featured by a third-party firewall;
- TransProxy (process is called "tproxy") listens on HTTP port 80 and 
redirects clients, once they accepted the AUP, to the third-party proxy 
via the LAN NIC.

Note that tproxy currently only supports the LAN interface and mode 
switch -p is hardcoded. Not yet supported are the -f (redirect to fixed 
URL, -r (run-as-user) and -a (ACL). But I intend to make -t, -f and -a 
available as well. I also look into supporting other interfaces than LAN 
(for just one single NIC other than LAN, it actually should already work 
if you specify the IP address of that interface as the bind address in 
TransProxy).

Feedback is appreciated.

Rolf