What did you have in mind? I think the only things you'd have to worry about are (A) you won't get hens and chicks on the bottom and (B) having the right kind of structure that provides some kind of soil for them to root in.

They don't have deep rots at all, and are VERY good about needing just a tad of water to get by. And Hens and Chicks / Sempervivums (among other species) are commonly used for green-roof projects of all scales (office buildings to birdhouses), so it should be fine.