Is there a way to get the old pre-Google+ username for YouTube channels using the API?

Using the API, I want to retrieve the pre-google+ youtube username of a channel, i.e. the string that comes afteryoutube.com/user/

I've tried using using v3's channel-> list API call but this does not return anything related to pre google+ usernames. However you can set 'forUsername' to filter for a channel with a username so I wonder if it's still supported.