I built a redeye tool for rawproc a couple of years ago, and the best algorithm I found was simply, for each pixel in a radius around the center point of the eye, calculate a new red value to be an average of the green and blue values. Gets rid of the red cast, but not at the expense of the overall tone needed for definition. There’s also a threshold of difference between red and the green+blue average, so not all pixels get the treatment. You can read about it in the code and comments, here:
This implementation requires the user to pick eye centers; there are ways to automatically find eyes, I think OpenCV has such algorithms.
The specifics of this approach would probably require either 1) persuading @agriggio to implement it; 2) use a tool like G’MIC, ImageMagick, or rawproc…
Really, the best way to get rid of redeye is to move the flash away from the lens centerline, and the only reason I even implemented it was because my primary camera, the Nikon D7000, has a built-in flash.