It occurred to me that there may be some mileage in using a monitor to locate, register the sprocket holes and provide a trigger for the camera. I knocked up a simple light trigger using a TLS2591 light sensor. This can provide several outputs including total light.
The sensor is controlled by an Arduino nano using I2C and switches an output pin high when the light is above/below a preset threshold. The sensor is quite small but, if mounted directly on the carrier, would obscure the screen and make alignment difficult. I used a length of 4mm fibre optic cable to couple the screen to the sensor which works really well. The mount slides along the monitor for X positioning and I incorporated bearings on the optical mount to enable Y positioning. A small screw locks this in place.
First results are quite encouraging, I had to reconfigure my main system to use an external input instead of the encoder. The software requires work to remove the second trigger from one side of the sprocket hole, probably some form of sampling to prevent false triggers and maybe tying in with the encoder system as a backup. Early days but I think this has potential.