Although there were stories of individual Lindens talking about this behind the scenes, there has been no official response, until now, when ProductTeam Linden finally made a post in the JIRA.
It's clear there is a lot of interest in SVC-7125. The intent was for users to specify their last name using the Display Names feature found inusers' profiles, which most Viewers now support. For those that haven't used Display Names yet, you can also set them on the web: https://my.secondlife.com/settings/profile.
Know that we hear you and value your passion and that we are currently reviewing some of the decisions that were made with the username /Display Names implementation.
While a canned response that was quickly answered, "We know that was the idea, but it DOESN'T WORK for the things about which we're complaining ... ," this shows it's finally gotten through to the Lab that they, or rather the "Resident"s, have a problem. Hamlet Au predicted Linden Lab would soon give those under the new naming system the option for a surname. No doubt they're hoping for this or something close to it.
Other articles: New World Notes, Daniel Voyager