Well, not sure what "gaurantee" that there needs to be, these are not unlicensed replicas or fakes, that are oddball cards and you can label them as such. You are only grading 4 things, corners, edges, surface & centering. Those can be done with ANY card that will fit in your cases. If I got an art card signed through one of your witnessed signings, you telling me you wouldn't slab it? All you need is the info on the card, and in my case, I did all the research for you.
Bottom line, CGC needs to expand their market to ALL cards. If you find them to be fake, THEN kick them back.