The ceiling is typically 2"-2.5" thick. The paneling on the inside is probably 3/16" plywood with some kind of fabric glued to it. Since most of the stress on a joint between the ceiling and a wall would be side or shear stress, screws would not have to penetrate more than 1/2" or so. If I were to to something similar, I would probably secure two strips of trim along either side of the wall and let the wall float between them. This should cause minimal damage to the ceiling in case you want to remove the wall later. But, is it necessary to build a wall between them? Would a curtain suffice?