Today, you could poll the BigBlueButton server using getMeetings and getMeetingInfo for the current state of the meetings.
If you keep track of this state, then you can detect when meetings start (they will appear) and when meetings end (they will disappear). If you poll every minute, for example, they you will know within one minute of creating and ending of a meeting.
Looking ahead, in the upcoming BigBlueButton 1.0.0-dev, your application will be able to register to get callbacks from a BigBlueButton server.
For more information see
We don't have any date for the release of BigBlueButton 1.0.0-dev, but we are working on finishing the development.
We'll post an announcement to this mailing list when it's ready for others to download and try out.
Regards,... Fred