So, from the MuM, it seemed that the web-proxy for RB was a new "re-write" from MT. That is they are writing it from "scratch" ... thus is isn't so stable. The web-proxy on the x86 is actaully some squid derived system. Lastely, you could always make your "own" transparent web proxy as djape said:
After that you can configure transparent web-proxy and do redirection in dst-nat.
Someone correct me if I missed something.
You can simply grab all the traffic that is coming through the forward rules and send them to your own squid server. Then special case the squid server so that it can actaully get out.