What I specifically want is to be able to have older, Ethernet-challenged Macs surf the web. (Most notably a PowerBook Duo out of its dock, so that I can just plug it into a PhoneNet adapter and long phone cord, and surf the web.)

The ability to do straight file sharing directly between LocalTalk and Ethernet machines would be a nice secondary benefit, but not as big a deal. (If I can get to websites, the local file sharing isn't as urgent.)