I agree 100% with Jeff, the advantage of this is that you could build it for cheap, very, very solid. The height it swings to will give you the energy with the formula E=mxgxh where

E=energy in Joules,
m=mass in kg,
g=9.81m/s^2,
h=height difference in m

This will be the total amount of energy transferred to the target. Unfortunately, this amount of energy is not necessarily representative of how destructive/fight-ending a strike is, as pushing it to that height slowly will require the same energy.

Knock-out power is also represented by the acceleration of the head in the few split-seconds directly after impact, this is what causes a concussion. This could be measured with a circuit that measures the time taken for it to pass two closely spaced points, the first of which should be very close to the initial position of the pendulum. The strike should go through such that the fist/foot goes further than the last of these sensors. If these points are very closely spaced relative to the arc length of the pendulum, you can use a=2s/(t^2) where
a=acceleration in m/s^2
s=distance between sensors in m
t=time in s

You would have two aims, arguably in order of importance:

1.) make it accelerate as fast as possible
2.) get it as high as possible

If you achieve aim 1 you may cause a concussion/broken rib, assuming you have enough of 2, meaning you would also need to push through the strike! If you achieve aim 2 with a poor aim 1, you will push away your opponent without causing any real damage. I guess I don't really need to expand on this as you are a karate instructor, apologies for going over things you probably already know, it's just best to be precise and complete :-)