bridge and PF for transparent proxy

[Mij]

Hello folks,

On a FBSD7.1 box I would like to implement this sort of
"transparent reverse proxy":

inet <--->  (vr0)<box>(vr1)  <---> host

such box is expected to
1) pass transparently anything from inet to host and viceversa
2) redirect some of such traffic (some well-defined TCP connections)
from "inet" to an application listening on 127.0.0.1 on the box
3) make this application connect to "host" pretending to be the
original source -- that is, using as source address the address
of the client that connected to it from inet

I use bridge(4) over vr0 and vr1 to implement 1).
I use something similar to
http://marc.info/?l=openbsd-misc&m=108089194621750&w=2
for 2).

Although from the network perspective 3) seems easily feasible as
well, I cannot think of a reasonable setup on the box host for it.  
Anyone
has some advice for it?