I just figured out how to enable echo cancellation for PlaceCam in Linux with PulseAudio. It is possible to PulseAudio configuration hints using environment variables. When using the command line, you can tell it to activate echo-cancellation like this:
Oleg and me tested this and were surprised how well it works. I hereby encourage anyone using Linux to adopt this method so the audio in our virtual meetings will be less painful.
PS: Open Research Topic: Find out how to automatically or at least super-conveniently (I'm thinking push-to-talk) mute the microphone when not speaking.