I had to get that one in but I like the vision which you are painting. A remarkable idea if it can be done. I think you should put three or four layers of chicken wire netting in front out of sync in order to reduce the size of the holes plus put in a layer of moss or coir to retain the growing mixture.Now what to use as a mixture which should be quick draining, retain moisture for maximum time and still be light weight? This gravel and sand combo wont do due to weight and anyway it will run out like water from a convenient place. What is the performance of these plants in a quick draining organic mix, any idea? You will still have to add some lime sand to keep them happy.
Regards,
Arif.