I live in the desert and sand/dirt is everywhere. A neighbor lost a fine stallion to sand colic so I'm a firm believer in keeping all hay in containers large enough that they can root around, toss the hay a bit and still not end up with it in the dirt. I'm also a firm believer in giving them something to help sand pass easier.
I've had so many people recommend beet pulp that I make it a regular part of the diet as an extra precaution, especially since I keep a couple of grass areas for grazing. Every summer the weather gets too hot and the grass gets short and stubby until the monsoon rains come. I haven't tried psyllium, but from reading, it also sounds like a good choice for this.
I've never tried sand clear but if its the product you like the sound of then its worth trying. Better to try it then have the worst happen.
I wish I could remember where I read about this so you could read it too, but theres a way to check how much sand your horse is carrying around inside. You take a fresh apple (thats what the article called horse poo) and you break it up into water and let it soak. When its dissolved enough for the poo to be loosely floating, then you examine the bottom to see how much sand is there. Unfortunately, I can't remember how much sand is considered acceptable. It said something about how horses are usually carrying a degree of sand in their guts and that it settles inside and part of it gets carried out with the feces under normal conditions. If their sand load is high, then it can quickly turn serious or they might carry it around without problems. But basically, the higher the sand load they're carrying, the more likely it is to become a serious problem. Even a well cared for horse can be carrying around a good deal of sand.
I tried this test on our pony and it showed very little sand, so for us, I felt that something was needed because of our desert home, but nothing aggressive.
Good luck with your search for info!