Hmmmm...seems kind of silly and unnecessary to me to go through the process of building a precision rectifier if the only thing you have to do is reverse the wiring. It's not as if you're going to completely destroy something in a Roland pad by opening it up...there's not much to it, it's still just a couple of piezo leads.
The other thing that I've been thinking about is how you get a positive or negative first half wave signal. For the DIYers, if you have the head piezo with the ceramic lead going to tip and brass to sleeve, rim piezo with brass lead to ring and ceramic to sleeve, you should be good to go.