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.
I found this thread trying to get this to work and was successful! Here are my settings that I think are relevant.
OctoPrint: 1.9.3 with original camera stack
Cam: Microsoft Studio Cam with resolution set to 1080 via octopi config file on boot.
Scrypted config:
- Installed the ffmpeg-camera plugin and added a camera via that plugin.
- In the "Extensions" settings panel for the camera I set to the following:
- In the "FFmpeg Camera" setting panel, I added the following:
FFmpeg Input Stream Arguments:
-i http://your.octoprint.url/webcam/?action=stream
Check "No Audio" at the bottom
More screenshots to come, but I'm newly registered and that limits me to one inline image. 
- In the "Snapshot" settings panel:
Everything else was default. I was able to add this to Homekit and stream live once this was in place. I'm using an nginx proxy in front of my octoprint server, but other than that it is pretty vanilla and I only changed what I posted above.
Apologies in advance if the formatting is a mess here with pictures inline and such. 
This works (thank you) but the camera that the HKSV Scrypted plugin generates for Homkit does not have any of the features that other cameras I own in Scypted have. No notification options in the Home app settings for it, for instance -- I can't record on it, etc. So I'm wondering if there are more settings to adjust to get these?