Trouble with multiple interfaces and rdr ?

I'm trying to setup a WCCP enabled transparent proxy and have been
facing various troubles, but finally got to a point where ipfilter
seems to have the key to the issue.

Basically, the redirected packages from the client are getting to
the proxy (squid) via a wccp(gre) tunnel, but the answers from the proxy
are going back without the proper de-NATing and so it does not work. :-(

This seems to be consequence of the rdr rules being attached to
interfaces, and the route to the client does not go through the tunnel,
which is the incomming interface for the client connection (and thus the
one with the rdr command).

Does this sound any bell to anyone?
Am I way wrong ?
Is there any way to fix this ?

Thanks in advance,

--
Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina