That's easy...you can do the same by just fretting a note but NOT playing it, and then start to move the note with very small rapid movements to start it first sounding, then playing loud.
Just sustaining a guitar won't get minutes of legato, but by 'trembling' the string you can get it to swell in volume again and indefinitely, not that hard.
Dee
"When life's a biatch, be a horny dog"
Amps: Marshall JVM 410H w/ Plexi Cap mod, Choke Mod & Negative Feedback Removal mod, 4x12", Behringer GMX110, Amplitube 3/StealthPedal
Half a dozen custom built/bastardized guitars all with EMG's, mostly 85's, Ibanez Artwood acoustic & Yamaha SGR bass, Epiphone Prophecy SG, Vox Wah, Pitchblack tuner plus assorted pedals, rack gear etc. for home studio use.