Yes, normally this is done on rooftop patios where the load limits do not allow for the weight of 2 or 3 feet of soil in the planters. In this case, all but the top 4-1/2 to 6-1/2 inches of the planter is filled with closed cell foam, then LiveRoof is set on top to give the illusion of substantial soil volume.
Read more about lightweight green roof implementation on our blog post.