What do you think of COLIBRI[1]? Does it seem like it could address this problem adequately? Does it have a chance of getting traction beyond just Jitsi?
That's interesting, and it's about server side multiplexing. But I have no clue why it's not accepted as official XEP yet. In general, the progress of XMPP standards seems very slow.
[1] https://jitsi.org/colibri/