Before reinventing the wheel I did look around for a tool like this. What I found was a market of crazy, obese, gold plated implementations that actually cost money and often often lots of it. I don't object to making money from software, but not from something so simple as this, that is something that requires hardly more than a single web page. And so the development began.
The webapp is in its very early stages of development. I have just touched the surface of the Google Calendar API and its use within a HTML/JS page. At this point I am only concerned with the mechanics and so have made little attempt to make the UI useable. My goal with the UI is to have a tablet by each meeting room running a browser in "kiosk" mode and only showing the meeting room page. The page will show the events for a user who has authorized and selected a calendar.
If you want a side project and are interested in working on this with me to complete it the project is at https://github.com/andrewgilmartin/meeting-room-occupancy. A live version is running at http://andrewgilmartin.com/meetingroom/.