Bulk Update Participant Status via REST API
The right place to experiment with possibilities of the API is API Explorer that you can find at Support -> Developer - API Explorer in your local CivCRM instance or on the Demo site. 

All the participants of an event can be selected with:

http://dmaster.demo.civicrm.org/sites/all/modules/civicrm/extern/rest.php?entity=Participant&amp;action=get&amp;api_key=userkey&amp;key=sitekey&amp;json={"sequential":1,"event_id":3}

The update of status can be accomplished with the create action in combination with an already existing participant_id. For example:

http://dmaster.demo.civicrm.org/sites/all/modules/civicrm/extern/rest.php?entity=Participant&amp;action=create&amp;api_key=userkey&amp;key=sitekey&amp;json={"sequential":1,"id":3,"status_id":"Cancelled"}

moves the status to Cancelled.

Success with your app.