Music annotation is an important step in several activities related to music transcribed in common music notation. We propose a protocol to annotate collaboratively such scores in real time. Based on a paradigm with commutative operations, this protocol guarantees consistency beween distributed editions while providing a fluid user experience, even behind possible network lags. It is being implemented into Dezrann, a web platform for sharing music analysis. We report efficiency and scalability tests on the current implementation, including usage by up to 100 simulated clients.