TOPIC: Users order in the Select a User dropdown

Quick question about entering a registration in the Registration Records screen in the backend. Is there a way to order the "Select a User" dropdown by last name rather than by user name? I have a few hundreds users registered on my client's website, and searching for somebody in that dropdown to register them makes it a pain... Because the username is not related to the last name or first name, is just something arbitrary that they select when they register the site. Any suggestions?