Like William 1 said, I would try and slice the homasote in the middle of the "belly" and then screw each side down with lots of screws to get it back to level. If the middle rubs together, unscrew and cut out a small (1/8") section along the slice and re-screw. Once set, patch over the slice and screws with flexible speckling compound.
If all else fails, perhaps you could cut out the "bellied" section and replace with a new homasote patch.
