I'd definately ask glenn about this, but if I had no info I would want to do it just like overlaying concrete. I'd drill some holes in the side of the shelf curve, glue in some wood pins or phenolic like mentioned above then just make a form and use bondo.
Or hell what about just getting a small piece of slate from a counter top manufacturer cut it to form and epoxy it on with the pins attached in the middle?
Glad you brought this up as my table brunswick classic plays great with the
4 1/4" pockets I did on it. But its a home model table so the shelf is not very deep at all, and have been thinking about extending them myself also.
Grey Ghost