so i made Live squeak good feedbacks at me.
audio FX channel receives input from an instrument on different channel, and sends to a return, that is routed "Audio to" this very same audio FX channel. its a feedback indeed.
on the FX Audio channel i put a bandpass auto filter with a limiter/compressor after it
---edit: bell shaped EQ (8) will do a even a better job to my taste ---
the return will be slightly overdriven with dynamic tube, saturator, and or compression/limiter
i really manage to get some organic feedback noises out of this basic setup. with a good control on the frequencies through some auto filter cutoff tweak..
if we could just have send/return routing option within an audio fx rack (!) this could have had been much more elegant...
have fun!