Did you get this working?
I'm in the same place as you.
All other methods of getting the OctoPrint camera into HomeKit suck, poor quality and/or laggy.
No. I couldn't figure out the right things to put in for the stream URL.
I was able to get an OctoPrint stream into Scrypted ffmpeg using the OctoPrint plugin "Camera Streamer Control" (I was using this for a webrtc stream at one point):
ffmpeg input stream: -i http://OCTOPRINTIP/webcam/video.mp4 (I'm not using any additional settings for the stream, but I assume the Scrypted ffmpeg plugin will work with those).
snapshot url: http://OCTOPRINTIP/webcam/snapshot
To be transparent, this is part of the new camera-streamer webcam stack discussed in A new camera stack for OctoPi
Ah yes, thanks for the clarification/correction. I did switch to the new camera-streamer webcam stack a little after that blog post (and apparently forgot that this plugin was part of that, rather than something I downloaded after the fact).
The webrtc stream would also be available to scrypted I suspect without the plugin. The plugin just makes the webrtc implementation of camera-streamer work within OctoPrint.