[Slow_Hand]

I like the thought, but trumpets require a lot of energy to excite them (i.e. you have to blow a LOT of air into a horn just to get a note. Getting an instrument like that to feedback would require a pretty radical system.

The difference with electric guitars is that guitar pickups are relatively sensitive and then go through multiple stages of amplification, which makes the system ripe for feedback loops.

Some saxophone players have been known to generate feedback through on-board microphones. Strictly speaking, this isn't exciting the horn, but it does introduce feedback that's excited BY the instrument.