How to build temporary walls for a temporary room
I wouldn't try to use drywall for this, plywood would be better, but it isn't readily available in 9' lengths.  You could build panels out of 1x4s like fence panels.  You could beef up and use barn door slides instead of closet slides.

Another idea, a different way to go - if you made five 9'x4' panels, 2x4 framing with 1/4" plywood on one side, you could use lag shields and screws to attach to the floor and the walls where they abut, and use carriage bolts and nuts and washers to attach them to each other.  Then maybe angle brackets at the top to keep them plumb and keep them from getting pushed over.  This would easily disassemble and reassemble with just a ratchet. When it's put away between visits, you'd just have the small holes where the panels attach to the walls and floor to look at - no major blemish.   

I think I'd build the walls, assemble them, THEN add the plywood the first time.  That will be easier to adjust if things are not perfectly square and plumb, which they almost certainly are not.