Very INGENIOUS!! I'm joining the 'popcorn' gallery to wait for spring & watch the progress. All I wanted is a root cellar, but your idea is REALLY great!
not sure if you have your roof figured out yet, but one earth covered home that I worked on used bituthane 3000 from grace company. it is perfect for below grade application, it was applied directly to 3/4 plywood which needed to be primed w/ a special primer first. I'd guess that materials for your roof would be about $300, from what I did last summer(07)
you can find a supplier from their website.
this home was huge timbers inside, covered with 3/4 white oak, 2 layers of 3/4 plywood and then the membrane, next was 4" foam, then a layer of poly, then 6" pea gravel, than 12-18" soil.