I've never tried that so I am not sure it will work, but when you've completed all the goals of the map, you can chose to go back to the main menu, or to continue playing on the map. If you keep playing, maybe you can keep loading sawmills as much as you want?!
Yay, this is something I’ve been saying for awhile. The ability to increase difficulty on maps. Increasing mill log requirements and maybe increasing fuel consumption. Seems like it could be a quick setup type deal before starting the map.