While not free and open source. A number of occupy locations have started using http://cel.ly as a way to broadcast important messages to occupy participants. Direct Action in NYC has an account set up to garner instant communication with movement activists. It might not be the best option, but perhaps it's better than relying solely on Twitter.
Below is the text from http://nycga.net:
Scan the QR Code below with your phone or text message "@occupyalert" to 23559 to sign up for text message alerts from the Direct Action Group to be alerted to join us in the park should another eviction or other emergency be eminent.
Standard messaging rates will apply.