I can't speak for 'Gladys', but that describes exactly what happened to 'Cavatina' off Durdle Door. It all happened quite quickly and was unexpected (!) so I didn't survey the scene in a 'that's interesting, I wonder what will happen next' way: but I clearly recall that I was surprised at how high out of the water the boat on its side floated, because of her side buoyancy which is the full length of the hull. She was tipped over by a wave plus wind with the rig to leeward, and the windage on about 2/3 of her rather plump hull above water would have been considerable, and working to invert her. Like a properly RYA-trained sailor, my first action was to swim to the stern to try to turn the hull round: had I managed that, I'm pretty sure the wind would have righted her, or made it easy to do so. But she inverted almost immediately.
The masthead airbag is a most ingenious idea (now why didn't I think of that!?), and seems to be quite compact until inflated. A bit pricey, but worth thinking about.