Yeah now that i think about it a part that ignores collision wouldnt be Professional, thought of some restrictions, like you can only build it in top of walls and are still able to walk on it, tough.
And for the hitboxes no big Change but instead of a giant cuboid(still using the coalplant as example), a rough outlining for the chimney, so like a square outcrop roughly the size of one tile( thickness and hight same as before but not the length) would do and also doesnt sound Impossible or even that labor intentsive to make.