It should be easy if USB networking is already set up.
Someone needs to compile iptables support into the kernel (they could be distributed as modules in a repo since we are all using the same kernel and compiler from nokia) and then port iptables itself (who knows how much/little work that would be).
From there it is trivial to forward traffic from one interface to another.

All that is needed is iptables...
Though there may be an even simpler way, I only know how to do it with iptables or ipchains