For a much better standalone app on OSX, check out https://github.com/kbhomes/radiant-player-mac (I just use this project as a workaround when Google changes things and Radiant Player hasn't been updated yet).
Google Music Keys is a simple utility application for controlling the google music player with keyboard shortcuts.
This fork changes the OSX minimum version to 10.7 as well as other changes when necessary.
Simply iterates over all open tabs in Chrome and Safari, finds the first one with the title Google Play Music, and sends some js to the page to click the relevant button.