Alex Lifeson from Rush is a killer at volume swells, check out his playing on exit stage left (1982 i THINK), Pettrucci does lots of it too -like on lifting shadows off a dream.
You guys heard of an e-bow? It's a different sound (more strident, richer in harmonics - much more like a violin bow) it makes a magnetic field over a string causing a feedback loop with indefinite sustain. They cost about 90 quid but they sound fabulous. I think Pettrucci's feedback solos probably use them quite a lot.