Yea, a noise supressor wouldn't do much. Let's say you have a channel with a mic running into it, and then out to the PA. If you place a noise supressor on that channel, it would shut the channel off when you aren't singing into the mic. When you start to sing, the noise supressor would open up the channel, and any frequency prone to feeding back would be projecting at the same volume as everything else.
Beyond placement (which doesn't always fix this problem), you could try putting a paramteric EQ on the channel, and fiddling with that until you find the narrow range that is feeding back, and then cut that frequency down.
To get more accurate yet more complicated, you could pickup a feedback supressor, but I figure you were just curious since you had the NS-2 sitin around.