To make this work, you'd need either...

1) The ability to adjust camera speed dynamically to keep up with changes to rotor RPM. You'd need a digital camera I think, unless the view finder looked through the shutter too.

2) A helo with an automated rotor RPM, ie autothrottle. All helos would have an ideal rotor RPM for a certain phase of flight, which you could know in advance. The problem with a manual flight control system is that any change to throttle or collective will change RPM, at least a little bit, unless precisely counteracted with the other control. Ground effect and airspeed would probably affect that as well.