Add settings fields on options discussion admin page

Problem description

I’m searching for a way to add a settings section/settings fields to the ~/wp-admin/options-discussion.php page exactly where Core calls…

do_settings_sections('discussion');

How to

When I register the setting and add the settings field, nothing appears:

function register_setting()
{
    register_setting(
         'discussion'
        ,$this->option_name
        ,array( $this, 'sanitize_setting' )
    );
}

public function add_settings_field()
{
    add_settings_field(
         $this->option_name
        ,__( 'Label', 'wpse_textdomain' )
        ,array( $this, 'render_field' )
        ,'discussion'
        ,'discussion'
        ,array()
    );
}

Debug/inspect

Now I was curious why nothing appeared and dumped the global that holds the settings section right below that line in core:

var_dump( $GLOBALS['wp_settings_sections'] );

The output was a plain simple NULL.

Question

How does one register a settting there, without registering the section with add_settings_section( 'discussion', __( 'Title', 'textdomain' ), 'callback', 'discussion' ); first?

Reason

The reason why I ask for “without registering the setting without the section” is plain simple: If another plugin registers the same section afterwards, it would overwrite/override the added section.

EDIT To make it more clear what I’m asking for. I’m aware that there’re two sections default & avatars already present on the page and that I can hook in there. Point is that I don’t want to hook into existing core feature settings, but below them, right into the settings sections that core should output with do_settings_sections( 'discussion' );. Point also is, that the discussion page was never registered by core and therefore can’t get used as do_settings_section() aborts without being able to loop through any (not) registered pages. I’m fully ok with an answer that confirms that and states: No, doesn’t work. Is a bug. or even better creates or helps creating a patch.

You have to call add_settings_section() first, pass a unique ID and assign it to the page (poor name) discussion:

add_settings_section( 'ads_id', 'Extra Settings', 'ads_description', 'discussion' );

Then register a callback to save your field(s) …

// Register a callback
register_setting(
    'discussion',
    'ads',
    'trim'
);

… and then register one or more fields:

// Register the field for the "avatars" section.
add_settings_field(
    'ads',
    'Test field',
    'ads_show_settings',
    'discussion',
    'ads_id',
    array ( 'label_for' => 'ads_id' )
);

Here is a very simple example:

add_action( 'admin_init', 'ads_register_setting' );

/**
 * Tell WP we use a setting - and where.
 */
function ads_register_setting()
{
    add_settings_section(
        'ads_id',
        'Extra Settings',
        'ads_description',
        'discussion'
    );

    // Register a callback
    register_setting(
        'discussion',
        'ads',
        'trim'
    );
    // Register the field for the "avatars" section.
    add_settings_field(
        'ads',
        'Test field',
        'ads_show_settings',
        'discussion',
        'ads_id',
        array ( 'label_for' => 'ads_id' )
    );
}

/**
 * Print the text before our field.
 */
function ads_description()
{
    ?><p class="description">This is the description</p><?php
}

/**
 * Show our field.
 *
 * @param array $args
 */
function ads_show_settings( $args )
{
    $data = esc_attr( get_option( 'ads', '' ) );

    printf(
        '<input type="text" name="ads" value="%1$s" id="%2$s" />',
        $data,
        $args['label_for']
    );
}

Note the fifth parameter for add_settings_field().

Result