online planner with move_base - checking for footprint legality

I am trying to build an online planner that interacts with move_base by iteratively sending next goals as a service call to move_base. Therefore I would like to find ways to check for footprint legality. 1. One way is to subscribe to the footprint and the global costmap topics and then construct the list of cells within the footprint and then check all of them for obstacle using the costmap. 2. However the world_model allows to check for footprint legality directly as is being done in the carrot planner. How should the world model be constructed in this case? Or is there a better way to do so? Thanks