There is no way to do this in CUCM. The whole idea behind call routing is that a user's call will route over an available path defined in the Route List/Route Group listing. The user should not care if the call is routed on-net or off-net. But with additional capabilities for on-net calls (like video), it will make a difference.
But based upon when the call will select the final gateway for egress is well after the call can be routed to a media resource to play the message you desire. Once call control lands on a route pattern with a route list/route group, then you cannot insert a media resource to play an announcement.
Thanks,
Dan Keller
Technical Marketing Engineer