Paul,
Pool filter sand is heavy enough that it is not shifted around, but fine enough that it forms a solid bed that keeps large pieces of waste from getting trapped. I think it also provides extra bio filtration- as the bb colonizes the surface of the sand and helps consume the fine waste/helps break down the large waste. My only question at this point is the long term build up of fine particulate matter. But as you suggested with your system, what is left behind probably does not contain much organic matter and a good vacuuming, either weekly or monthly, should remove much of what is there. I think in combination with the other elements of your system, it would not be an issue and may even enhance the filtration system. I'm no expert and welcome correction on this matter. I have just ordered an artificial stump as a centerpiece decoration. The pointed ends are all facing down, creating hiding places for the fish and eliminating potentially dangerous contact.
http://www.drsfostersmith.com/produc...m?pcatid=19344
I'm starting to enjoy the hobby again now that I've gotten past issues of disease and water quality. I'm willing to service my micron filter and purigen every couple of weeks if that's what it takes to keep pristine water between water changes. The combination of the micron filter, the purigen reactor, and the sandbed for bio filtration have made my water and tank more attractive than its ever been. The water at the surface, under the LED lights is like clean glass.
I think I will monitor my system before adding any new elements. I like the idea of having dedicated canisters: 1) Pleated Micron Filter, 2) Purigen Reactor, 3) Bio Canister- sand or media. Looks like those moving sand bed filters would be a cost effective way to create the bio filter and the purigen reactor. My mag 350 already has a micron filter.