If anyone else is curious, the Wave protocol is also open source, as is Google Mobwrite (which uses differential synchronization instead of operational transformations (which is what Wave and I think Etherpad uses)).

My "drawings" are represented by text (JSON), and if normal text merging methods will work, I might want to use differential synchronization (seems easier). But I wish there was something proving that it won't mess up the integrity of my structured data.

Links:

Wave protocol: http://www.waveprotocol.org/ and http://code.google.com/p/wave-protocol/source/checkout

Mobwrite: http://code.google.com/p/google-mobwrite/

Etherpad: http://code.google.com/p/etherpad/

you can use a file synchronization algorithm. here's one I did for JSON:

http://blog.mozilla.com/rob-sayre/2008/02/15/restful-partial...

It works well except when array elements move around (that works, it's just unintuitive)

I didn't know about sync.js - this seems like exactly what I want.