I've never seen a plan for this, but I have (maybe) some related experience. We had a ping pong table top for our last pool table. Without going into my opinion on whether this is good or bad, with our ping pong topper had stiffeners across the inner surface (part facing the slate) to keep it from sagging. That didn't work too well, and a ping pong table doesn't have a lot of weight put on it. I suspect that a dining topper would need an undercarriage that sits on the slate (the felt) to remain solid, and it would also need cleats to keep it in position (our ping pong topper had them fit into the corner pockets. I could see such an arrangement crushing the felt where it sits, and that might make it a little tough to play pool. The strongbacks on our topper weren't stiff enough enough, but you might be able to make them more stout, or even develop a torsion box type of top that wouldn't need them. Such a topper would have to be in at least 2 pieces to allow easy removal for use as a pool table (ours was 2 pieces), and that would also mean a seam where any spilled drinks/liquids could leak onto the felt. Give careful consideration to doing this, it will take some thought and planning to avoid ruining the pool table. It may be possible to cover all the felt with something like a piece of ply/melamine/mdf, then put the supports for the top on that, that may prevent "tracks" on the felt, just think it through and insist on a damage waiver.

 

Googled it and there are lots of images of commercially available toppers. One thing you'd need to consider is room for people's knees and chair height. You may get to build some benches as well.

 

Wouldn't it be cool to have the top split in half and be on tracks or a bracket somehow to stash down to the sides ? Maybe someone has designed a bracket for this. That way you wouldn't have to pick up and carry a big top, especially a nicely made, heavy one, and ditch it somewhere while you're playing pool.