Vegas summers are rough! Usually the hot weather tapers off mid Sept to early Oct. So the first weekend in Sept. might be a crap shoot. I would suggest planning some short shaded routes as a back up (First 4 pitches of Dark Shadows 5.8 should provide a nice place to climb. The Dark Shadows area gets a little sun in the morning but stays shady thru the hottest parts of the day) Stay away from East facing climbs like Solar Slab (aka sizzle slab) if the weather is "hot" you will bake!
I don't know off hand when it is shaded or has some sun. I climbed it last year mid-summer and it was shaded and cool. Your rope will get wet! but if you can attach it to the outside of your pack it should be dry by the time you get back to the car. I'll ask around this weekend.
Dark Shadows gets shade about 1pm in the summer, probably a bit earlier by September. If you wait till afternoon, you can get on other routes on the east face of Mescalito,such as Cookie Monster or Y2k. We just had a lovely afternoon on Y2k this week. While the valley temps were over 100, Pine Creek weather felt wonderful. We got a late exit permit and had a nice hike back by moonlight. It is worth noting that the hike in to most of the canyons is in the brutal sun, so getting in as early as possible is helpful. Canyons not on the scenic loop, can be reached earlier, since you don't have to wait until the gate opens at 6am. I freeze about 3 quarts of water in a gallon bottle and hold it next to my core if I think it will be uncomfortably hot. That keeps me cool enough.