as for the compression test idea...
although it is a nice idea, it falls down because the inlet valve is still open when the piston is well on it's way up the bore. So by the time the pressure in the cylinder starts to increase the volume is already much less than the swept volume. Maybe if you knew the exact cam timing of the valves opening and closing (not at .050" like cam specs are usually quoted at) you could work out where the piston is when compression starts, and then the volume at that point.
Then you can use simultaneous equations to solve the unswept volume bit.

Brain hurting yet? Basically it's a whole lot of maths and a little bit of physics to work out something that probably just isn't worth it... I would just rip the head off....