Welcome to my world....
Seriously- just do the best you can in these situations. When I record concerts, I have to do a lot of strange things to try to deal with room situations that are "less than perfect." I think the worst was about 3 or 4 years ago, I did a concert recording at USC in the recital hall. It was composition recital so there were ensembles that ranged from solo piano to chamber orchestra. As it turned out, Nelly was giving a concert in the quad right next to the hall. At the steps of the hall, it was 120dB SPL- literally, the entire building was shaking from the massive amount of sound.
Due to the fact that most recordings of live events are documentary in nature, we do the best we can and then just accept the problems. If I can record a dress rehearsal for editing, that helps (especially when it is for radio broadcast). HVAC can sometimes be filtered via FFT or sometimes even looping room sound and phase inverting it. When it is time to do a "serious" (aka released) recording, I get much pickier about rooms. I will usually scout the location to make sure that things like noise won't be a problem. In city locations, I will often insist on recording after about 9:30 or 10:00 at night.
I will also bring lights if the house lights buzz. You can get cheap flood lights that can take 100w bulbs at Home Depot for just a few bucks. I keep a box of them around for sessions.
To avoid light and other creaks, I set the lights at the beginning of the session and leave them. To avoid dimmer buzz, I'll turn lights full up or completely off. Lights will often make noise when they heat up and cool down so they will be turned on early enough so that they are at full operating temperature before the session begins.
Even with all of the prep work, there can still be issues. I did a recording quite awhile back in a silent location. (see here-
http://gearslutz.com/board/showthrea...&threadid=6424 ) The room was in the middle of nowhere. In the afternoon, we started to hear dogs barking- quietly, but audible. When we went outside to see where they were, we saw 2 dogs fighting with each other close to a mile away. We just made sure to take plenty of starts and endings so we'd have quiet room sound.
--Ben