I rigged one up a couple of years back. It was nothing special, but it worked, kinda. If you actually put some time into its really not that hard. i modified a stock shifter with a Lokar cable kit, its basically just like setting up a normal clutch, except the clutch is on the shifter. I simply mounted a pivoting lever on the shifter and when i would grab the shifter i would be grabing the lever, and the shifter togeather, thuse ingaging the clutch. I thought it would make for quicker shifts, but inless you can come up with a way to have very little travel in the lever it is actually slower than a regular clutch (if you are good a shifting). Have fun experimenting, thats all I did. Let me know if u come up with anything that works.
PS - I have never seen one done before (besides mine), and I have never seen a kit either, so maybe you can be the first and make a lot of money off it!