Thanks for the reply @simon and explanation of the existing feature.
Our site may be a bit of a special case, because we do not want to have the user’s Wordpress username synced with Discourse precisely because Wordpress does not allow users to change their usernames by default. And we do not want to install a plugin that adds this WP username edit functionality due to the instability that it could create.
But since usernames are editable in Discourse, I think it would make sense to extend this functionality into Wordpress via the WP Discourse plugin. This field seems like the perfect place to do that, but I understand that it could conflict with the WP username since that would never change by default. So I take it that this is not something you want us to contribute to the plugin?