Must encode into memory. The MediaEncoder that comes with Unity would serve my needs but all the constructors require a path so it apparently directly saves to a drive. I neither need nor want that and of course I need the developing video file in memory so I can send it frame by frame.
And MediaEncoder is, unfortunately, Editor only. Having better access to low-level encoder and live streaming is something we want to work on, but it is still under consideration. Be sure to go vote on the roadmap and leave any other ideas there. It helps us to prioritize future features Audio and video workflow roadmap | Unity.
Until then, you have multiple choices. The asset store has some live-streaming solutions like AVPro. Otherwise, GStreamer is an open-source multimedia framework, and I think I saw some integration with Unity somewhere. It uses an LGPL licence which comes with its legal issue, so beware.
Thanks for the suggestions. I cast a vote for runtime encoding (and runtime decoding). AVPro has a trial version so I will check that.