Here's the problem with that method.

The pseudo-PPP dialup and regular dialups require the server, not the client to provide PPP services.

I am pretty sure there is a way to provide PPP services over WiFi, just like there is PPPoE, but the ISP isn't a PPP provider.

Furthermore, Ethernet is significantly faster than serial ports are. The fastest transfer rates I observed from these machines was 3.5Mbps, but that's over AFP. FTP is a little more on the slow-side. http://www.applefool.com/se30/moreinfo.html#transferrates