Why not just purchase an inexpensive Multimeter and use the Ohm/Buzzer setting.
Plug the 2 leads into the meter. Clip one lead to the cutter spindle.
Attach the other lead (Permanately) to a thin metal plate and your good to go.
That will give you an audible indication when the cutter connects with the plate but has no feedback to the control board to stop the z-axis movement (or any other axis that you would probe on). Therefore you would have to do a very slow axis movement with very little steps. I doubt that this is faster or easier in the end than just doing the same with a piece of paper.
If you want a cheap and easy solution, take the same parts minus the Multimeter and connect the leads to the SO controller board like described in step 3 here. That will allow you to probe with a simple G-Code that takes care of everything.
And then you could replace one probe with a piece of metal and the other probe with an alligator clip attached to your end mill shank, and you have a touch plate.