I only tested one hop, e.g. A(10.x)---->B(192.x). All I had to do there was to adapt the routing tables: On A, route 192.x traffic to the "natter" tap/tun interface (I always forget which is L2), and on B, route traffic to 10.x accordingly. That's all.

For it to be routable in the entire network, you'd need to obviously mess with a lot more :-D