I noticed that LibrePlanet is going online-only this year on very short
notice. They will certainly (try to?) use only Free Software, maybe we
can learn something from that...
here's the solution FSF came up with. The main conferencing tool was jitsi, which the organizers used with presenters. They then recorded that and created a stream of it.
I don't know anything about jitsi but at a quick glance it promises to be a full Sip and web-based video conferencing tool, supposedly with no accounts required which would be surprising.