For the record, these things already exist. Here's an example of one. Similar devices exist for Ethernet as well.
To clarify matters a little bit, these are not general purpose "network cards". They're essentially a modem of sorts designed to tunnel RS-232 over TCP or UDP to another computer. For instance, you could use one wired up to an old dumb terminal to connect to port 23 (telnet) on a UNIX server as if you'd dialed into it over a phone line. (The "dialing" process would involve sending whatever escape sequences the thing uses to bring up its connection menu, using that to open the telnet port, and then dropping back out into data transmission mode.) Another (more typical) application is allowing the use a device like a serial barcode reader remotely from a server without having to run special wiring.
It would in principle be possible to use one of these to network an old Mac... the simplest way would be to set up a UNIX-y server that would accept a tcp connection from the device and offer a script which would start a PPP or SLIP session. Then on the ancient Mac you'd use an appropriate PPP client to "dial" into the server and go. But the thing to remember here is the device will *not* just act as some sort of magical network bridge. (For instance, you won't be able to browse Appletalk shares by sticking this on your Mac and joining it to your Airport.)
To clarify matters a little bit, these are not general purpose "network cards". They're essentially a modem of sorts designed to tunnel RS-232 over TCP or UDP to another computer. For instance, you could use one wired up to an old dumb terminal to connect to port 23 (telnet) on a UNIX server as if you'd dialed into it over a phone line. (The "dialing" process would involve sending whatever escape sequences the thing uses to bring up its connection menu, using that to open the telnet port, and then dropping back out into data transmission mode.) Another (more typical) application is allowing the use a device like a serial barcode reader remotely from a server without having to run special wiring.
It would in principle be possible to use one of these to network an old Mac... the simplest way would be to set up a UNIX-y server that would accept a tcp connection from the device and offer a script which would start a PPP or SLIP session. Then on the ancient Mac you'd use an appropriate PPP client to "dial" into the server and go. But the thing to remember here is the device will *not* just act as some sort of magical network bridge. (For instance, you won't be able to browse Appletalk shares by sticking this on your Mac and joining it to your Airport.)