- core_user_get_users_by_username is not going to implemented because it would be good to avoid to have several get_users_by_field functions. I personally would prefer to have one get_users($field). Maintainance should be much better this way.
- sorry to not have alerted you in my first answer that there is no need to contribute core_user_get_users_by_username. I remembered about it when I searched in the tracker for the function name (MDL-28606).
- I don't know when MDL-29938 will be released. I'd like it in 2.4 but I can't promise anything.