I've written a simple new plugin that provides a dropdown of locations next to the Longitude and Latitude boxes. When you choose a location from the dropdown, it fills the the coordinate boxes with the predefined values.
This hack uses the same wp_edit_advanced hook that Exhibit uses to hook into the advanced editing interface. If you don't already have Exhibit installed, you'll need to add a line to your advanced edit page to get this plugin working. Details are in the install file.