Connecting to the serial port has been done before by someone, but it's not an easy process. Basically it would require you to build your own stick controller interface.

The tuner module route is definitely easier because it has the stick controller interface built-in. Of course the would require you to buy a tuner module, either by buying one on eBay (probably expensive) or waiting for the tuner module kits that Patrick has made to become available again. The tuner module kits are basically DIY kits and require you yourself to solder them together. (there are people on this board that offer to do this for you as a paying service)