I think this is unnecessary and adds complexity - simply having the most recent change override another change is probably much simpler and perfectly acceptable… a song is a big playing field - the odds of two players colliding is really rather minimal.
Have you ever worked two people on a spreadsheet in Google Docs, at the same time? It’s a fascinating experience, you should try it … The tracker is basically a spreadsheet, so you can get a feel for how this could look and work with a spreadsheet in Google Docs - you can see the other “player’s” cursors (in different colors), you can see their edits in real-time, you can see them enter and leave the session, and you can chat with them. If you could see a ghosted version of the other collaborators’ mouse pointers too, that would be awesome. If the cursor left a subtle trail when changes were made (that fades or disappears after a few seconds), that would take some of the “spookiness” out, so you don’t suddenly realize that someone made a change where you were looking.
With the low latency on modern internet connections, you don’t need to worry much about the lag - participants will see the other editors’ cursors and pointers, and they will naturally get out of the way. I’ve been working with 4 different people on the same spreadsheet (relatively small, perhaps the eqivalent of 3-4 patterns in size), and we never once collided or overwrote each other’s changes. I don’t think manual “reservations” are necessary or pratical.