See https://news.ycombinator.com/item?id=3403924 about "Hadoop Reaches 1.0" almost a couple of years ago.

Is it possible to build this (or any other 2.x) with homebrew, or do I need to build it manually?

Assuming you mean installation (not build).

Don't think you can install with homebrew (yet). Apache Ambari, another apache project that helps install and manage Hadoop stack, should very soon be supporting installation of 2.x both on single node and on a cluster.

Apologies, I did mean install. Thanks for the tip- I'll check Ambari out.