We did something similar a while back, sending data from a browser extension to everyone who was using a Meteor app, without being logged in. Back then, we used this: https://arunoda.github.io/meteor-streams/
Even if it hasn’t been maintained, as with most Meteor.js stuff, it should work without any (or with very few) modifications.
Meteor could be used to share the ID between the clients, I guess? And as far as I understand WebRTC supports many-to-many.
All of this is highly theoretical and I have not implemented this anywhere, I was just wondering out loud if the latency incurred with a round-trip through the server might not be the right approach to share mouse pointers in real time
I agree, latency would become an issue if it goes via server. It sounds like it could be a candidate for WebRTC, particularly if security is not a big issue and all that is needed is disseminating IDs to participants. Interesting to hear about the outcome if @thomastraum gets to implement it.