Have you looked at the weather forecast for the time of your performance? That will give you an idea of how cold it will be, at least in the ballpark.

I agree with the statement to find the pitch center the band can best match. Sometimes you just have to tune the whole band low. I think ensuring the band plays with good intonation is going to be more important than a few out of tune moments between the pit.

My personal opinion is that judges know and understand when very cold weather creates tuning issues between the pit and rest of the band and should be able to overlook it in their judging. There's not much the pit can do to change the pitch of their instruments after all. As a judge, what I'm listening for is if the band overall can still play well in tune and adjust to the cold weather.

That being said, if you can alter the pitch on your keyboard, I would go ahead and do that because...why wouldn't you? Some keyboards will let you lower/raise the pitch or you could use the wheel, which obviously would be a little trickier for your keyboardist unless they have experience with it.

What stinks is when your band goes first thing in the morning when it's frigid, and other bands have later times when the cold doesn't create so many intonation problems. I always feel that puts you at a disadvantage, but, again, not much you can do about it.