I'm not sure about making calls on a card already in a slab. It can be done, but I'm not certain if it can be done reliably and profitably. The comment posted elsewhere about 15 min/card makes me think the price wouldn't make it practical for most cards.
But
Besides the group doing something with slabbed coins the stamp hobby has a fine tradition of expert certificates, some more reliable than others (Slabbing was tried by PSAs stamp division and has been a failure) And it's typical for some higher value stamps to come accompanied by multiple certificates.
I do think there's room in the grading business for a premium service. Not merely slabbed with a somewhat arbitrary number assigned, but a truly premium service that would slab -Or perhaps not slab, or review while in a slab, and supply a detailed written report. Expensive and time consuming certainly, but for very expensive cards it might be worthwhile. Such a service could also use a registry that allowed multiple companies cards to be entered, lowering the need for crossing over and the risks that brings.
Most other mature hobbies have fairly comprehensive national clubs that provide a number of services - magazine, authentication/grading, group rate insurance and educational. Compared to stamps and coins the card hobby is like herding cats.
Steve Birmingham
And just for clarity- I collect both slabbed and unslabbed, but can't see the point of slabbing vg or lower commons from common sets.