[maxmsp] avoiding feedback with extra long reverb

Hello, all.
I've written a piece that requires the use of an extra long (30+ second) reverb, and as can be expected, I'm having feedback problems.
I've tried delaying the original signal by a few seconds before it hits the reverb, and setting up a pair of buffers to alternate recording and playback duties, creating the illusion of a continuous audio feed, albeit delayed by 1000 milliseconds.  Any ideas?  I have limited time in the hall it's supposed to be performed in, so right now I'm just sitting at home with a mic in my hand trying different feedback killing techniques.
Thanks!