The only problem with trying to feed Sand Clear in a mash, is that it should be fed dry. The purpose of the product is to absorb a bit of water from the intestines to form a gel-like substance to move through the intestines to remove the sand. So, if you put it in a wet feed, that will completely negate it's effects.
I know they want you to start off with 5 ounces, but maybe introduce it little by little with his grain?
I, too, feed wheat bran on a regular basis and all my ponies seem to be fine. I don't use pounds and pounds, but 1 cup a day for a mash. Especially handy and nice on those cold winter mornings!