I think the idea is not to use a crossbar at all, but to just hang the paper taut between 2 stands. You'd need some weights/bags on the stands and you'd also need to stretch it pretty good, but it might be possible.
That's asking for trouble. I don't think you would ever get it flat without some kind of rigid support at the top. That's what the roll provides with regular use of paper.
I'm going to give away a big secret here, something I've been working on, but you should look into placing a smaller rod/pole inside two larger diameter pipes. Read: two seven foot pipes with a nice solid connection between and inside the two. Depending on the strength of the join and the length of the span, you might need to rig up a center support.
Oh yeah, have you checked out the location yet? Something tells me you could figure out something easier.