Hi Andrew,
I contacted the most knowledgabe guy I knew at MS about your scenario and this was his reply:
"For phone numbers dialed by UC enabled users, you could configure a route to a particular gateway. It would be up to that gateway to do something meaningful with the number.
For SIP URI’s and calls by non-UC users, you might be able to create a static route to do the trick."
So it looks like there could be a way to do something like you want, but nothing that is a standard or even an advanced setup.
Also, with a normal phone system a mis-dialed call is a mis-dialed call and is rejected. I will include this suggestion in the product feedback and see if it is something that can be worked on for future releases.