Way to mount a crystal ball on a stair post finial

Wear.

The overlays appear to be solid hardwood. The plywood is a thin veneer over other non hardwood plys. As the steps wear, you run the risk of wearing through the veneer. Even if you don't, if you need to refinish, you again risk sanding through that thin top layer.

In my old house, the bottom stair had a worn dip in the middle (reputedly eroded by my younger son's butt that perched there in his many time-outs). A veneer would have been long gone.

It looks neat.

I think I'd be looking for a glass railing engineer (as in an engineer employed by a firm in the business of glass railings or engineered/architectural glass structures in general) to ensure that the glass, mounting and attachments were all adequate. And I like doing stuff myself. I'd just be uncomfortable with the possibilities for building something that looks (as this does) like the sort of thing you'd see in a mall, airport, etc but that then fails to perform as required when it matters - and that could be from something as minor as the way the holes for the supports are put in, or tiny defects on the clamping surface of the mounts (either of which might result in cracks, and cracks lead to glass structures falling apart.) These folks (and the correct hardware, glass, etc.) exist - as witnessed by the malls, airports, etc...

One thing might be to beef up the top rail into a more structural component.