Check out this gearbox, http://www.rghelmstedt.de/joomlag/ 

The only info I can find is this on the site:
Transmission:
5-speed sequential racing gearbox housing in the original Beetle
Ignition interruption during the switching operation
Limited slip differential

I don't think it would be cheap, but how good would a 5 speed sequential gearbox be!

Hell yeah! Can do subie 5 speeds as sequential but the cost is prohibitive. You have to run dog gears so you can change without a clutch ($3500) then need to add the sequential mechanism with ignition cut on change ($4000). So a $10,000 gearbox. Not too many keen on that!