Replacing rp_filter with iptables

I've had to disable rpfilter for an interface (tun0) for an OpenVPN network which uses SNAT and policy based routing (see http://michael.gorven.za.net/blog/2008/ … ort-number">http://michael.gorven.za.net/blog/2008/08/15/routing-port-number for example). Several sites I found (none of which I can find at the moment, of course), suggested using iptables to mimic the functionality of rpfilter (in my case, I have a packet that was originally sourced from 192.168.1.100 (b/c of the default route) being SNAT'ed to a source of 192.168.2.5 (b/c of the policy route), and rp_filter was rejecting the return packets).

I didn't find any instructions anywhere for how to implement rp_filter (where needed) using iptables -- any advice?