This brings me back to when I boarded that a place w/ no electricity or arena lights so when winter came the only time I could ride was in the mornings before work. It also had only porta-poties, so I had to change at work. Thankfully my office had a bathroom where I could lock the door and have total privacy rather than trying to strip of tall boots and breeches in a bath room stall (impossible!)
Cover as much of you as possible, gloves, long-sleeved sunshirt, cap or bandana. Since the heat is a problem, can you ride earlier in the morning when it is cooler out thus making you sweat less?
The products I always had in my ‘quick change’ bag
Batiste dry shampoo was a total lifesaver. its like $6 and you can get it at some target stores.
makeup remover wipes - cleans better and less sticky than a baby wipe
scented lotion - its not as heavy as a spray, depending on the brand
deodorant.
totally fresh change of clothes - including undergarments and shoes.
makeup bag - putting on some makeup after scrubbing my face with the wipes me feel like the transition from barn to business was complete.
I also totally agree with have an opaque nail color - it can be a neutral tone as long as its thick enough to hide the barn scuz!