Okay... although I'm not sure why you picked this dongle necessarily. RS-485 isn't really what you'd want to interface to the Mac's serial port anyway. It's technically RS-422, but lacking the ability to do SDLC (IE, Localtalk) you're limited to RS-232 speeds/protocol. (USB adapters that support SDLC are mucho, mucho bucks, we had a thread about those recently.) There are plenty of microcontrollers (chips/complete boards) that have RS-232 built right in. (Well, you might need to add a dingus to go from ttl to rs-232 signal levels when you're talking about raw microcontroller chips.)
In any case, if you're talking about sticking this in a Duo with no USB the presence or lack or drivers for this thing for OS 8 is immaterial; you'd have the *serial* side hooked up, not the USB side, and the drivers are for the USB side. If you did stick a little microcontroller-controller-WiFi dingus interfaced via serial inside your Duo somewhere (the guts of something like this, for instance) then you'd need to use apropos software for it to set up a "serial" network connection, which would probably translate to using a PPP dialer to "dial" into a computer somewhere on your local network over Telnet.
In any case, if you're talking about sticking this in a Duo with no USB the presence or lack or drivers for this thing for OS 8 is immaterial; you'd have the *serial* side hooked up, not the USB side, and the drivers are for the USB side. If you did stick a little microcontroller-controller-WiFi dingus interfaced via serial inside your Duo somewhere (the guts of something like this, for instance) then you'd need to use apropos software for it to set up a "serial" network connection, which would probably translate to using a PPP dialer to "dial" into a computer somewhere on your local network over Telnet.