I have a simple way to make a continuos fiber extruding hot end that cuts the fiber each time you stop the flow. Basically, you would just need the nozzle to be made out of hardened tool steel (carbide would be better but good luck with that). Inside the nozzle you basically have a stopcock where the rotating part of the valve is also made of hardened steel. Each time you close the valve, the fiber gets sheared by the valve. This also takes care of nozzle leakage and it might be able to even out the inconsistancies caused by varing filiment diameters by leaving the valve partially closed while extruding, causing a back pressure in the hot and, and since molten plastic is a compressible fluid it would act like a hydraulic damper, evening out difference in pressure caused by slight inconsistancies in filiment diameter. I definately plan to try this scheme when I make the hot end for the 3d printer I am building.

You might be interested in this thread: anti-ooze nozzle, and these reprap blog posts here and here.

Actually, some of those blog posts where inspiration for this idea. I never saw the forum thread talking about ball valves though. I was actually thinking "a ball valve would really be a great way to stop ooze ", because it does not force any material out of the nozzle when it closes, then I realized that a ball valve could also make a great fiber shear if you designed it right. In fact, I think it could be designed to draw some material back up into the nozzle as it closed.