Optical perpendicular speed sensor required
Your mouse linear motion sensor could be done. What reliability you need is not addressed. Optical mouse sensor modules compatible with microcontrollers are available:



This ADNS3080 Mouse sensor chip combines:


optical sensor array
array processor
digital communication output


The array processor processes image frames, and updates X &amp; Y position registers. When you read these registers, they're reset to zero, so that your microcontroller can calculate speed (it is up to the microcontroller to measure time between successive reads). XY array could be aligned so that one axis (say X) is aligned with conveyor motion. Y-axis should then read near zero on average. Or XY axis could be aligned with 45 degree offset, so that X and Y registers yield similar numbers.

The mouse chip itself has no optics, so a simple external lens is required to focus an image of your conveyor belt onto its internal optical array. You'll also likely want to illuminate the conveyor belt with a lamp.
Some speed calibration will be required, since the image size is affected by lens magnification, hence the motion registers will yield different numbers.
For this module, standard serial communication is used to configure, read and write mouse-chip registers.
Many of these chips can deal with video-speed image processing with enough illumination, but there will be an upper limit to the conveyor belt speed sensing.
This is likely not a robust way of controlling conveyor speed, but as an open-loop sensor as you have outlined, seems workable.