Thanks for sharing those pics. I'm re-inspired now about adding a roof rack/deck. It looks like that red flame & blue bus has freshwater tanks up top (the long black tubes), which I am also considering as a passive solar water heating system. I also like the spiral staircase on the back of that one bus. Several of those buses have decks/rails look really rickety though... yikes!
I know wood is easier for many people to work with, but I think at least the structure of my future roof rack/deck is going to be metal. I might use wood decking, but even then, I'd rather have some sort of expanded metal decking, which I would assume is lighter weight, more durable, and will not catch the wind as much.