Could you use moss, like they do in the baskets? If you plant on of the tiny sedum or semps they don't take much in the way of soil.
This sedum is growing on a cement bridge over the creek in the back yard.
Here is a cute miniature sedum called lydium. it would make a ground cover in that gazebo area.
This Sedum 'Tri-color' would drape over the edges.
And here is a link to miniature sempervivum that need very little soil.
The thread "Meet the Miniature Sempervivum" in
Sempervivum forum
