Another Firewall Builder CookBook chapter tries to reproduce rules for transparent proxy with PF found in the document OpenBSD Packet Filter (pf)
The same rules can also be found in many other places on the web, for example here: http://schools.coe.ru.ac.za/wiki/Configuring_transparent_proxy
These rules can be reproduced exactly for the most part, except for the inbound interface matching in the redirecting NAT rule. Firewall Builder rule model for the NAT rules does not provide place for interface so this can not be done exactly like the original requires. However the rules I propose match source address of the packets to achieve the same goal.
See new Firewall Builder Cook Book chapter here