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