pbxn,
if your branches have pbx systems at them which connect to other pbx's via voip, or p2p, you may be able to connect an analog trunk to the system, and route 911 via that line.
i work with the avaya ip office mostly, and i can set a system wide short code at each pbx system to dial emergency feature, via another trunk group when 911 is dialed, which has the analog line connected to it just for this.
all other calls go out via the normal route on the normal group. dialing 911 is actually a system short code which then replaces the called number with 911, but sends it out another trunk group.