Sounding better and better!
Personally, I like the idea of steel over wood or aluminum. I'm sure if you used the right diameter/thickness it would be safe enough, but I'm not a materials engineer, so I'd rather over-design.
Whatever you decide to do, I'd strongly recommend rigging up some kind of safety rail. Do a Google search on this site for "bunk safety rail" and you'll get a number of good ideas (and a few off-the-wall ones!). I like Steve's idea to extend the post a few inches above the mattress so you have a solid place to attach a rail. That could also be adapted to the Scamp
set-up if the posts were cut a bit taller. I believe there are some important guidelines about the minimum and maximum gap between the mattress and the rail. Don't want kiddo's head getting caught in there! I recall reading about it in one of the threads on safety rails. I'm sure you'll run across it if you do the search.