In my case I wanted 3/4" OSB as it has similar mass to 5/8" drywall. That way I wasn't giving up mass to have the convenience of attaching anything to the walls at any location. In your case if your only concern is some columns on the walls then I would probably stick with 5/8" drywall. Like Big said, glue and screw to drywall can work pretty well (see Bacon Race theater). I might be a little (just ever so small) concerned that the drywall might crack along that edge but I'm definitely not a drywall expert so someone else may have to chime in on that. I can say that two layers of 5/8" drywall with green glue in between makes for a very solid panel and I think you would be hard pressed to get much flex in it on a 9" overhang.