For a lo tech approach look up www for info as nearly all meteers will have a sweet spot where an external maget rotates where a 'dry contact' external magnetic reed switch or mag HES can be placed safely.
Optically you could experiment with the following sort of an idea. With a bit of software you could detect the slightest movement or change movement of a digit with my sort of general purpose dual LDR '1 pixel' camera. I use a picaxe since it is fabulously simple / ratiometric ADC and just a battery and single chip job Add a simple ASK radio module to remote it direct wirelessly into a Pi ttyAMA0 serial port
Placing the LDR at the bottom of black poly irrigation sprinkler stalks drilled out to 5mm (LDR dia) allows for a tight focus or locus of interest: Ant movement at mm or a sheet of A4 paper being waved several meters across the room while rejecting ambient light level changes, ranges, artificial light flicker etc since the mid point resistance / voltage ratio is immune to absolute light level.
With a bit of tinkering and the readadc10 command 10 bit ADC it is easy to detect the slightest movement 'DIFFERENCE' and being ratiometric it is almost completely immune to ambient light level fluctuations. (needs daylight / ambient light of course) I have made speed traps for ants etc with this. (it even detects the Direction)
But proof is in the eating.... This system could pick up the rotation of my power meter tiny wee dot on the rotating disc of the utility AC power meter from cm away through the box outside window