Don't know about that, but... I bought a 2yo filly from NCSU on their annual sealed bid auction this week. Picked her up Tuesday afternoon with the 14' BP stock trailer. She had never been in a trailer let alone been hauled. I figured she was going to be a nut... Opened the big back door, about 15 seconds later she stepped up and in, took the halter off, fired up the truck and eased around the barn, waiting for her to EXPLODE... She never did anything more that hollar a couple times at the mares in the front pasture. I stopped and started several times so she could get her legs underneath her before we left the barn. We drove about 60 miles back home, some of it through downtown Raleigh,N.C., part of this was on the interstate, 30 miles of it was two lane... She never hardly moved a muscle, eating hay, looking about, watching traffic, not bothered by the tractor trailers passing at 80+ mph, standing in the center of the trailer looking out the rear.... She never turned and looked toward the front. I could see her the whole time through the windows in the front of the trailer. It seems that they do like to ride backwards... Maybe the folks in the U.K. are really on to something!!! Our new AQHA Registered Appendix filly is settling in nicely!! First QH we've ever owned.

So far as your windows... You'll have to load her up and take a look. I know all of our horses are short enough that I don't think it would be a problem... Tallest may be 15.2. I know some of the Sundowner drop down windows are so large that it looks like a horse could jump or crawl through them.

Most horses if given a choice will ride backwards in a trailer.  It is doubtful that she will pop out your window if it is built properly and closed properly.  I would think it would be easier to do with their head than their butt!