If you are feeding adequate hay, it will do as good a job as giving any sort of fiber supplement. However, that (nor fiber supplements like Sand Clear) will prevent sand colic if horses are kept in sandy conditions and fed on the ground.
I'm not sure just how much metamucil you would need to give. You should compare the instructions and the amount of fiber in Sand Clear to the amount of fiber in Metamucil and try to dose so that you are giving the same amount.
Licensed Veterinary Technician