I used to think it was a bit like bug abuse to store things in sign posts but then I realised that the sign post actually represents a pile of materials, if the graphic was changed I don't see anything wrong with using it for storage. A windmill is a 6x6 square, until you build it, its mostly just a big pile of rocks, if you're paying rent for those 36 squares I don't see why your rocks should disappear. If you've got an issue with it then change the graphic or give us a pile of stones building, can you not handle a little abstraction?
Edit:I realise you plan to implement storage solutions, but why make this change before then?