Not having worked with camera Control/ Interactive Link between External Recorders and such to Cameras myself, i just had a look at the Z-Cam, cause i know that one would need to do that.
https://www.slashcam.com/news/single/Atomos-Ninja-V-Firmware-Update-ermoeglicht-ProRes-R-15954.html Given that it is apparently unable to communicate with the Camera when RAW is Recorded, which obviously should use way more Bandwidth than standard HDMI, i guess they are just using some kind of Ethernet Capability of the HDMI Standard here; not only to do the Communication back to the Camera, but also to Push the Additional Data for RAW Format here back and Forth. In case of RAW Recording that "spare Bandwidth" the HDMI does not take up will be used for Image data, so it wont work there.
https://en.wikipedia.org/wiki/HDMI#HDMI_Ethernet_and_Audio_Return_Channel I however highly doubt, that anything this in depth about the HDMI Standard or the Communication Protocol used here would be fully Public (have not spent more than 2 Minutes Looking tbf., so it might, but i dont think so. Its just like Camera Lens Mounts and such, usually you need to Buy the Full Specification and Documentation and sign an NDA to not make it Publicly available in any Capacity, Maybe even also a License to use it if for a commercial Product.).
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Your best Bet would Probably be to figure out, if you can somehow get hold of Blackmagic or Atomos Customer Support, ideally an Actual Person to directly Speak to, and ask them on how to do that. Othewise (if you cant find any more info online), your only other option would most likely be to find a way to extract and Analyze the Ethernet part of the Signal that is going on with these Recorders and Reverse-Engineer that, however one would do that (more of a Hardware Guy myself, i try to avoid coding and Signal Processing stuff if i can...).
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But in Short, Probably some Kind of Ethernet signal.
Might not even be super Complicated, should just be some sort of command/ Status update the Camera gives out via HDMI if i understand correctly how that would work for start Recording on Record Button press at the Camera.