At work, we have a system that sends out IMs and texts to all employees at the site (usually used for weather alerts and once for a chemical spill). I don't know if it is a homegrown tool or COTS, but searching for "Desktop Enterprise Notification System" turned up a bunch of results that might help.

It looks like a lot of them are service providers instead of just software providers. Maybe something like MissionMode would work for you?