Oh wow, every single one of you starting from Nedko have just saved me hours of research! Thanks so much guys, extra points to Andy for his walkthrough with bonus code, Folderol for citing the exact reference material and Josh for landing the correct device (eroktronix) =P

Yes, it's obvious yet I didn't contemplate the piezo-DI-soundcard route. I feel so stupid now, but thanks to the prompt response here I have something to work on right away. Initially, and also the reason why I was so lost with this is because, I wanted to bypass a full-fledged microcontroller eg. Arduino (since I'm only dealing with one channel) but still have MIDI via a very simple breadboard _before_ reaching the computer. Credit this to my lack of understanding.

Yes, I'm sticking the piezo onto the skin/head internally. As for the beater hitting directly, I don't know if that's harmful but it's worth a try. I'm positioning it behind the skin opposing the surface area where the beater hits, instead of placing it "off-axis" somewhere else nearer to the rim (common when triggering externally). In any case, I've got to get the signal up and running before I can actually toy around.

I like the threshold-detector-software suggestion, but as mentioned, it'll be a safer bet going with Arduino as most of the electronics are taken care of.

This list is awesome.