I'd recommended this a million times before, but the optimal solution is:
To have the creator sign a comic-sized piece of clear, flexible plastic/Mylar and have that encapsulated on top of the sealed and graded comic.
Why not just have stan sign the slab with some sort of ink that does not come off slab material?
oh sure, you won't be able to add $1000 to the price or whatever, but you'll know stan's signature is real and it won't screw up the book. if someone thinks that means you need a new slab, well i guess you can't please everyone.
I think you just came up with a new service - Slabbing Slabs!