First off, I am not sure this is legal in the US, but it would be fairly simple with the SX-200, but since it has to be done via ARS, you would have to loop an analog extension back to a trunk, build a trunk group for that "line", and use COR to limit 911 to calls to that specific trunk group for those people...
Something like make a list of two routes, one includes the regular outside lines for 911 calls, and the second option includes the trunk group that contains the "lines" that are looped back to extensions, then use COR to restrict the rooms from accessing the first route, but allowing access to the second route, then use digit modification on that route to remove all digits and dial the extension you want it to go to.