This might be a bit late, but we flew our cats out of Dallas in May 2008. We had them booked on BA, and were aware of the heat embargo. About a week before the flights we realized the chances of it being too hot for them to fly where high. We decided to make a provisional booking with Delta as well, which at the time was on the list of approved routes. They didn't have an embargo, as they took the cats to the plane in air conditioned vehicles and loaded them onto the plane right before take off. They told me BA was just too "lazy" to take these precautions. On the day of, it ended up being too hot for BA to fly the cats so we used our Delta booking.
I noticed KLM is an approved route from Dallas, have you contacted them to see if they have the same heat embargo?