I ended up just coding to the OBS source code.
Good riddance to this pile of garbage in as many places as possible, and I don't see what it was doing in curl to start with.
RTMP has SO many users for video (Twitch, YouTube etc...) yet you have librtmp which has so many forks. OBS has its own version in-tree....
Then at the same time you have people trying to add extensions to RTMP still!
RTMP has held back the use cases I have cared about since ~2015 so I am excited to see people embrace other options.