They are placeable objects, and hence don't have animations like creatures do so they can't be possessed and used as a traveling vehicle. I also have an interest in making something like this happen though, so if we don't get modelers to make custom vehicle 'creatures' for us I may try a hack something by applying a 'Model' Visual Effect on an invisible creature and then possessing the creature to make it look like the vehicle is moving. It will be a bit wonky since there won't be any animations like turning wheels, but for my purposes it may be good enough.