Currently it is possible to stack a weapon on top of any other unit by using the save/load feature. Saved games don't keep track of where the weapons destinations are properly, if you tell it to move somewhere else while in flight already and save (before it actually changes direction) the saved game will keep it's original destination, but you can place another building at that place before saving, so when you load you end up with a weapon flying on top of another building and landing. It's also possible to stack more than 1 weapon like this, but quite tedious.