I wouldn't recommend it...
When tuning you push the car hard to find its maximum, then you take out a little bit as a safety margin to accomodate for hot days, slightly different fuel, etc... If you tune on a crazy hot day then the maximum is going to be lower, and then you're never going to be sure just how much of a margin to take out... My last tune with Trent was fuarken hot, think it was a 40c day. His ventilation is pretty good, but it was sucking in the air from above his factory through the roof which was still pretty damn warm so the car was getting really hot and pinging off its tits.
If you can reschedule, I would... but who knows their schedule and when they can fit you in next.