Thanks for the tip about railwaycat's fork. Note though that before you can run:

    brew install emacs-mac

you first need to run this:

    brew tap railwaycat/emacsmacport

Otherwise, `brew` won't be aware of that version. (Whoops. Noticed just now that someone said this earlier as well. Since the comments are all spread out, I'm leaving mine in case it helps someone see the issue faster.)