The blasting grit is mostly clean, unlike play sand. You rinse it to get any residue off it as some folks have mentioned in other threads. Mine was just dusty. I rinsed it in a bucket a few times and it was ready for the tank.
Eco-Complete, you just open the bag and put it in. You don't need to cap it, or use it as a cap. But there are enough folks who have done what you're proposing and others who have done it differently. All have had results growing plants.