As a Floridian of 25 years who HATES the heat and the humidity (sigh), I would run a 5k in the summer if it were held late at night. It never actually gets "cool," and the humidity never goes away, but not having the sun bearing down on you does make a huge difference, and a 5k is short.
The problem is the daytime. I live nearby and could drive up in the afternoon, check in to my resort, have a nice dinner, maybe take a nap, run the 5k, go back to the resort and take a shower and go to bed, sleep in until right before checkout, have a nice breakfast and maybe visit the gift shop, and then drive home, but I am NOT going to the parks in summer, AP or no AP. I don't know how many people would be willing to come to Orlando just to run a 5k unless they're park people who don't mind the ferocity of Florida summers.