KB9MWR,
Certainty, I have a Buffalo WHR-HP-G54 running v24-sp2 (08/07/10) std (revision 14896) connected directly to my cable modem. To the Buffalo device, I have a WRT54G v3 running v24-sp2 (08/07/10) mega (revision 14896). Prior to the v3, I also had a ver5 device running DD-WRT micro with no issues as well.
Connected to the WRT54G is a Ubuntu-based VM Server containing the virtual machine running my Linux AMPR GW.
You may wish to allow VPN passthrough under Security > VPN Passthrough; though this hasn't been needed in my configuration. I noted you said that you are not passing IPIP rip packets, as far as the DD-WRT router is concerned, IPIP packets are the same, no matter what's in the packet (I'm receiving AMPR IPv4-in-IPv4 in addition to IPv6-in-IPv4 for another machine).
To clarify, my WRT54G is not in the DMZ.
-KB3VWG Lynwood