First off, amazing job - the UX design is outstanding. I've been wishing for something like this for music composition for the longest time, something that would combine the ease of use and live-collaboration aspects of Google Docs with custom workflows and object types as you have here. (Unfortunately, the presentation layer for the music domain requires implementing an entire DAW or notation interface on top, both of which are herculean efforts that I haven't tried to tackle yet!)
That said, I'll probably end up using this for lyrics! (When I used to write for musical theater in college, our writing workshop would basically use Google Docs, using screenplay formatting and typesetting in all caps for singing. This would have revolutionized the workflow there, even without music-specific features. Individual songs have "beats" and evolving themes just like anything else!)
On the tech side, did you use something like https://github.com/jahfer/othello for OT, or roll your own? And do you think in hindsight that CLJS was the right language/toolset for the job? I've always been interested in its ability to treat immutable data as a first-class citizen, but I haven't had a chance to do it yet.