The graphic equalizer isn't that hard to deal with. The blue line on the screen is a series of rubber bands and you can click and move the curves up and down and make new ones as needed Here's an illustration of a curve that makes voices a little more harmonious.http://kozco.com/tech/audacity/voiceEq.jpg
You can tell where to put that dip by doing an analysis.
Put some feedback performance on the timeline. Select some of it.
Analyze > Plot Spectrum.
Do this on a computer with a really big screen. Pull the spectrum window out as wide and tall as you can. It will give you more and more detailed information as you go. Feedback, contrary to your assumption, is not a festival of frequencies. It's the room ringing like a bell at one frequency. The analysis should tell you where that frequency is. It should look like something that would give you a flat if you drove over it.
Mess with the equalizer to get rid of that one frequency.
There is one more interesting thing you might try. Everybody thinks Noise Removal is just for air conditioner trash or background hiss, but you can train it to eliminate anything.
Find a portion of the performance where there's feedback but nobody singing. This will be a little rough.
Effect > Noise Removal
"Train" the tool what feedback sounds like and it will try to eliminate it from the whole performance. The trick is not to include any voice in that training, or it will try to eliminate the voice, too.
There several reasons why none of this is going to work, but I haven't heard the show, so we'll watch and see how you do. Let us know.