Under each extension in the Advanced tab there is a field called Emergency CallerID. You can put the associated DID to be used for 911 there. Then you create an Outbound Route for 911, choose “Emergency Route” and do not add a Route CID and make sure “Override Extension” is disabled.
When someone calls 911 this route will match and because it’s tagged as an Emergency Route the outbound callerid macro will look for the extensions Emergency CallerID and use that.
Way simpler than you might have thought.