FYI, I created an OpenCV 3.x input plugin for mjpg-streamer that allows you to capture from OpenCV and filter the input via a separate plugin. More information on
github. And of course, there's a filter plugin that comes with it that allows you to write a python 3 script and use that as a filter, or you can write your own in C++.
Haven't done much testing with it, but it seems to work on my desktop. I have not pushed a working build into roborio-packages yet... I have packaging setup for it, but want to test it on the RoboRIO first. Will do that tonight.