I would be concerned about attaching that much weight to the MDF board. Also these ZOB's are heavy enough as it is, making a stone baffle would require 4 people to move one of them  ;)

Also I don't have a clue how much the stone would resonate and the ZOB requires resonance to work properly, Steve could probably help with that answe

Nevertheless I am intrigued by the idea and if you do it please post pics