Hangout-auto-try-again is a project mainly written in JavaScript, it's free.
Auto retry joining hangouts!
An "Auto Try Again?" will appear when the hangout is full. When clicked, you can try again automatically and it will auto join when there is a free spot.
The minimum delay is 5 seconds, but you can increase that in options. This number has been set this so that you wont get banned by Google and not stress the Google Hangout servers.
(https://plus.google.com/116805285176805120365/about)