From what I've heard/read, clamping from the balcony is the way to go at the Bowery Ballroom. What you described (clamp with extension pole) is the way I always clamp to balconies if possible -- clamp as low as possible and put out an extension arm about 3 ft out. I put it lower to minimize the crowd noise of people up in the balcony, which have greater potential to ruin your tapes in my opinion and experience. Usually the people on the floor are low enough to not have such tape ruining potential. Of course, YMMV. Good luck, I wish I had gotten tickets for the NIN show too!