FCC labeling rules - repackaged tablet
First FCC ID's are mainly for products with transmitters.  Most other products use a DoC or Declaration of Conformity - which is a pretty easy process.  In essence you need to be able to prove upon request that your product meets the requirements the FCC as placed on it.

Typically, I would say that a radiated emissions scan of the product without the clamshell and battery, and a second with both, and assuming they pass or at least are identical, kept in a file (forever) would be adequate to protect you from any issues with the FCC.

I would feel safe stating such in a manual - "This product conforms to the requirements..." or some such legalese, as stated on the FCC website.  You could add a label to the unit as well.

The main concern the FCC and all of us EMC type people have, is that your additional battery could cause an increase in radiated emissions from the unit.  You are tying a wire to the inside of a possibly noisy environment, and creating a passive antenna to radiated that energy.  That would need to be correct or incorrect somehow.