JP's piezo tone in Budokan sounds very much like a nylon guitar... Is that what you're looking for?
YouTube - Dream Theater - Hollow Years (live in Budokan)
Oh... Coz when I first heard that song, I thought JP was using a classical guitar of some sort...
I think with the right EQ setting on the guitar, the piezo can get pretty close to the sound of a classical guitar. When I was testing out an Egnater Mod 50 with my MD JP, I plugged straight into the amp with its Twin/Deluxe module. Frank Lamara, Bruce Egnater's partner, stepped out of the room just before I switched from the magnetics to the piezo, only to turn around and say that he wasn't sure it was the same guitar, cuz it sounded like a classical.