hehe, sustain is always an interesting subject, but along the way, there be many suggestion of extending the sustain by altering the characteristic of the instrument thru compression, ebow, sustaniac pickup, fuzz, delay etc
anyway, feedback also can have pitch one. If at certain angle/distance away from amp speaker, can get certain feedback with certain pitch, use it in part of a song when you are playing the guitar parts, stand round that area/spot, hit the note on your guitar with the same pitch as feedback, the note sustain will kinda fade off and with the feedback taking over at around the same pitch.
Matt bellamy from muse seem to use his fuzz factory in some songs kinda the same principle, exception its with fuzz oscillation within the fuzz factory(instead of feedback) and leading into the notes intro of songs etc.