You can always buy an old midi drum brain, this will simplify things a bit. You can use home made piezos on long cables to go into the brain (some have calibration), it will then give you midi notes out with the "volume" encoded as note velocity, super easy, convenient and no heavy coding or other software. As the piezos eventually die, I would send them with a bunch of spares pre-made so they are easy to swap out if the stop working (we were using them as cheap drum triggers, after a lot of hitting they would become less sensitive. You would probably have to design some kind of calibration routine in Isadora to get the setup correct for the show, but this is also pretty easy.
I used Alesis trigger IO (very old, but they worked fine) and you can still buy them- like this bargain
http://www.ebay.com/itm/Alesis-Trigger-IO-Digital-Recording-USB-Midi-Interface/182158567941
Fred