I'm not quite certain I understand the problem you are attempting to solve; but to add, I am actually passing traffic through two residential gateways/routers running DD-WRT. They do in fact pass IPIP, as I'm running an AMPR gateway and a 4-to-6 tunnel behind the devices.
Many newer consumer routers pass certain tunnels and allow multiple VPN connections which are not TCP or UDP (e.g. 'VPN Passthrough' on newer routers).
73,
Lynwood KB3VWG