Currently to send a command, you can do it towards one device at the time through MQTT. That kills the whole idea of publish / subscribe model. Would be nice to allow users to use some custom topics, e.g. "custom/notify", so that multiple devices can be subscribed to it, thus making it easier to communicate to multiple devices.
NOTICE TO EU RESIDENTS: per EU Data Protection Policy, if you wish to remove your personal information from the IBM ideas portal, please login to the ideas portal using your previously registered information then change your email to "firstname.lastname@example.org" and first name to "anonymous" and last name to "anonymous". This will ensure that IBM will not send any emails to you about all idea submissions