I was thinking something similar to what collo suggested but slightly different. If you happen to have a woofer with a level part in the middle, like
this one, you could just attach a thin mirror (aluminium foil might work) to the level dustcap and point a laser to that mirror at an angle. Then read the laser dot from a scale on the other side. Determining the excursion in mm from the reading you get from the scale is just geometry. See attachment for a quick-and-dirty scetch of what I had in mind..