A lot of stuff to go through here, so I can't totally guarantee my accuracy, but the original question seems to be looking for what's known as "send to self" technique. Linked search shows what I think is the best maintained kernel patch as top link + discussions and patches with other approaches on various mailing lists, esp. LKML.
I think one should also look at network namespaces, done with iproute2's "ip netns". This also takes some extra interface and routing magic, so might not even be less complex than the massive iptables hoopla in the other answers.
Comments definitely welcome if anyone found something useful with these - the how, the what, the where about your implementation.