I've used soft agar overlays before for 'phage, but not swarming bacteria, so I'm just thinking out loud. It seems to me that there's a potential problem that the occasional bacteria will land on, or very near, the surface of the agar and swarm from there. The phage I've used would spread less effectively on the surface. In the case that this is a problem, hopefully it is not, you might try to let the first overlay set for a few minutes, then add a second overlay with no bacteria to submerge the first one. If they swarm more effectively on lower percentage agar, then they'll really enjoy swarming on top of the top agar overlay.
Just a thought.