Yahoo uses akamai technology (bgp path analysis) to determine what
content server is "closest" to a given host and bases its' dns response
on that data. You could contact Akamai and, explain your network, and
see if they can make the necessary adjustments, or if you are large
enough ask them to drop a couple of servers in your site. (or they may
want you to send them a BGP feed)
It is a bit more complex that BGP path analysis. :)