fernandez made a "sustainer." i don't know if it was in the pickup, or if it was its own device. i know its activated by a switch. umm, kerry king has something like this too, if i'm not mistaken. i suppose something like this could bring out feedback. also, on the gt-6, there is a "feedbacker" setting. it sounds like a lot of this is used on deliverance. and, on the newer version of the ebow, there is a feedback/harmonic thingy. it sounds like this was used for the melody at the end of closure. so, there's some fucking feedback info for ya.
*edit* oh...didn't read ruby eyes' post.