I have been writing a Mac IP Gateway also, but mine will run on a Mac that doesn't have Appletalk (Tiger, Leopard, etc).

It works fine for statically configured MacTCP or OpenTransport assigned addresses, but I still have work on the dynamic assignment.

I use the bpf mechanism for better or worse and have the subset of AppleTalk implemented.

I would be willing to share source code with anyone that wants to see it. The only think I ask in return is help improving it.