Spontaneous ejaculation is common in horses so Is it possible that prostate play could get the fluids moving if done right? Yes, especially if the horse is already aroused, belly slapping, edging to the point of being flared. Will a full on orgasm be possible every time. Probably not. Will prostate stimulation alone work for every horse, No. Some may be more sensitive to and aroused by it than others, some may like a certain technique, like a circular thrusting motion with your cock, or even a vibrator held in a certain position, you just gotta learn what each horse likes and go from there, how erect he is and body language will tell you if he likes it or not. Considering male on male sex is a pretty common occurrence in equines, far more common than one would think (just do a YouTube and or Google search if you doubt that) you should have luck. Just exercise caution approaching a horse from the rear so you don't startle them and get kicked.
Btw that Welshpony video is hot and a perfect example of the fact that a stallion can enjoy anal. He doesn't reach a orgasm but that hard and flared cock dripping pre while he gets his ass fucked speaks for itself