There was a thread here a long while ago asking about if there was an adapter out there that could allow the Classic Controller to be used as a USB input without going through all that Bluetooth + Wii-Mote + Glovepie stuff. Mayflash has made exactly that. It's a two player adapter for the Classic and the Pro, with pressure gauge L and R's for the original Classic. Check out mayflash.com or browse Ebay, they are around $14.
Despite how it looks in the image, the housing itself is actually
pretty small, about a 3rd of the size of the Classic Controller.