Concrete Dome Homes

These look like a viable option.

Radon Gas and Water Vapor Sealers for Concrete, Brick, and Masonry

I am certain these must not be MCS safe products, but if you have time & space for off-gassing...  Since radon gas can be sealed, it seems as if this barrier should be able to seal any remaining untreatable mold.  The above-linked site also has deeply-penetrating products for waterproofing.