On a somewhat-related note, I was at a show in August and one of the supply vendors had a holder called "Graded Card Empty Slab". Here is a picture:
He gave the dealers a few free samples of his wares and after examining one I thought "why would I want that?" After reading the OP, I guess it could be useful if you wanted to make your own custom card display holders. Maybe cut down an old SGC black insert and use it for a T card...or maybe just for my kid's Little League cards with a DIY flip.