By the way, I still think my original idea would work, and probably much simpler to implement - but I should highlight one weakness of that approach: it interferes with undo/redo. That is, if you undo an operation, you’re not undoing an operation done by you, necessarily, just the last operation performed by any of the participants, which is probably not the desired effect.
For that matter, if an OT approach was used, undo/redo would be considered operations in their own right - that is, you would have to implement operational transformations for the undo and redo operations. I could be wrong, but I’m not sure the scripting API is up to the job - this may require tighter, more low-level integration with the Renoise editor than is possible with the scripting API. Or not - I don’t know it that well, and definitely have not been keeping up with new developments in the past 6 months 
If anybody has the time (and gusto!) to attempt something like this, here’s a very good explanation of OT: