Well, since you're on the 'Build Your Own' forum, why not get out the screwdriver and see what's inside the controller? If there is a potentiometer in there, you might be in a good position to hack the controller. Find out how many ohms the potentiometer is. If you are lucky, the potentiometer will be a 5K type and you can use it with a USB dual strike hack. (See 1UP's website) If the potentiometer is a 100K (gameport) type, check out Jude's Star Wars Yoke Hack (see the 'controls' section on arcadecontrols.com). If the potentiometer in there is some funny value, you can choose to remove it and replace it with one of the two values above and proceed with one of those hacks.
This should be a pretty straighforward project if you're used to dealing with electronics. Feel free to post or PM me if you have any questions about this.
-88mph