We needed to be able to re-use custom links (i.e. linking to "Google" on many of our posts/pages) and didn't want to have to continually re-enter that link as a custom URL.
I've altered the plugin locally to do the following:
Add "Custom" to the list of options in the settings page.
Query the database for all "custom_" meta definitions in the wp_postmeta table and present them as options to the user in the list
When you click on a previously defined custom link, it adds it to the page just like a normal post or page link.
Because I'm strapped for time, I've done this kind of brute-force. I can send you the code for you to clean up and review. This is a cool addition to the plugin as it saves lots of time if you have repetitive custom links to add to your pages.
Reply back with how I should send you my changes.