Maybe you could use the "milk" method ?
For metal objects, one thing you could do is use the method used in the Roland Modela Desktop CNC machines for calibrating the tool height. It works like a simple closed / open circuit : a current passes in the tool and the object, whenever the tool touches the objects, it closes the circuit. This way, you can have your tool move on a grid, and make it go down slowly at each intersection of the grid. Whenever you touch the object, you have an X/Y/Z coordinate, and with a fine enough grid, can reconstruct a relatively accurate object.