i don't know if you're trying this but perhaps you should try volume swells. Put the master volume and gain way up on your guitar, play a note with the volume down and turn it up until it starts to feedback. Try harmonics, they feedback more easily.
If you want really controlled feedback buy an ebow, they're a bit expensive but they've got a wonderful sound. i used to hear some ebow and wonder how the hell anyone got feedback like that; the answer, they cheated.