It's almost completely open source[1]. The only component that isn't open is the web interface itself, but there's complete documentation on the internal data structures, how to run a super node, and how to reconfigure the clients to serve up those super nodes instead of the ZT-provided ones.
I rather happy about that, it saves me a lot of work. I've already cloned the repo on github (and a few local machines). Essentially I currently plan on unwrapping their administration system and switching to a shared secret model.
[1]: https://github.com/zerotier/ZeroTierOne