No, the cracks can't just be filled in.
I took my dash pieces to a competent auto upholstery shop (Universal Auto & Marine Upholstery of Costa Mesa, recommended to me by Blumax). Universal re-covered them using a stretchy vinyl that is not backed with fabric.
This is not to say that any upholsterer could do this level of work - it took Universal some time (and it cost me some money) to get the job done. But, the result looks great.