Good question. The problem is that the AS granule size was made to allow for good CEC and c02 penetration to prevent compacting. So, it was not really meant to be covered. I am not sure if decorative ADA sand is an option as cap or even large enough no to fall thorough the Aquasoil,but you can contact ADA to inquire.
The only other thing that I can think about is something larger grained with good CEC like Soil Master Select or Aquarium Plants substrate, or perhaps Schultz Aquatic Soil(but that is butt ugly). Also SMS and Aquarium plants substrate which is alleged to be the same as SMS could mess with your water parameters.