I have computer A: IP 192.168.1.64 router 192.168.1.254

computer B: accessible straight through the Internet, IP 123.1.2.3 gateway 123.1.1.1 (switched, has its own IP, eth0)
Computer B also has a wifi network 1.1.0.0/16 and it's 1.1.7.3. (wlan0)

By doing ssh 123.1.2.3 from computer A and using -w 0:0 I have linked A into B's wifi network, using:

A> ifconfig tun0 up 1.1.10.1 pointopoint 1.1.7.3
B> ifconfig tun0 up 1.1.7.3. pointopoint 1.1.10.1

I also did:
A> route add -net 1.1.0.0 netmask 255.255.0.0 gw 1.1.10.1
so I may access B's network from A. Also:
B> arp -sD 1.1.10.1 wlan0 pub
to let them talk to A.

A is now effectively on B's network.

However, I'm now stuck. I want to use tun0 for the Internet, and as such I need to connect through the tunnel, into B's ethernet device as well as it's wireless.

How can I do that? I'm assuming you have to use iptables and routing?

I hope I made myself clear as possible.