I thought I'd experiment a little with some pressure-sensitive Velostat/Linqstat to see how it responds to pressure and whether it would be suitable for use as a MIDI pad/key as I'd seen a few questions about that but not many answers. My software is terrible but as a proof of concept I think it has legs so I thought I'd put up a little demo of my setup.
When you strike the pad the sampled ADC values increase over time to a peak and then start to decrease (a sort of bounce from the top) so my code keeps track of the highest recorded ADC value and sends a note on event once the input values start to decrease, using the peak ADC value as the note velocity. The note off event is sent once the sampled ADC value decreases past a certain threshold and the code starts tracking peak ADC values again.
A more reliable and tactile button might be constructed by using a conventional rubber dome push button on top of the Velostat pad, that way you could use the button for reliable note on/note off triggering, with the Velostat being used to detect the velocity and handle any note aftertouch events.