Oh yeah outline might be good. This is definitely something that I would not have been able to do for the jam but it fixes the problem mostly. Though we use a different layer for each object (actually sorting order) so they are each displayed according to the last time you placed them. This layer also corresponds to the position of the object as a child of his parent. We use the events system to drag and drop but I'm afraid Unity handles that in a very shadowy way that I would not be able to explain. Maybe using a raycast and comparing the int value of the sorting layers should be more efficient (but would hurt a bit bit the frame rate, not by much). )This is definitely something that I should look more into details for the future.
