For this, a workaround would be to use a normal sprite to do the resizing, and have you set the X/Y of the 9patch to an imagepoint on the sprite.
Do this and then adjust the width/height of the "menu BG sprite" (normal invisible sprite). Since you can set the origin to the center top you should have no problem with that. Then when the "menu BG sprite" is being resized, you resize the 9patch the same way, and then set it's X/Y to the appropriate imagepoint on the "menu BG sprite"
This should work as a workaround for your issue, but should work until there is more hotspots on 9patch.