[DX] Form API: Allow specifying alternative configuration settings for elements using `system_settings_form()`, without having to use custom submit handler

In https://api.backdropcms.org/api/backdrop/1/search/system_settings_form it mentions how you can specify a different config file for some of the form elements/values via '#config'. What do you do though if you want to save to the same top-level '#config', but to a different setting?

So say that your form element is $form['my']['cool']['element'] = array( ... ); but you want the setting to be saved as my_cool_element in the .json? ...is a custom submit handler the only option in that case?

So basically, I understand that you can do this:

$primary_config = config('mymodule.settings');
$secondary_config = config('mymodule.moar.settings');

$form = array('#config' => 'mymodule.settings');

$form['first_setting'] = array( ... );
$form['second_setting'] = array(
  ...
  '#config' => 'mymodule.moar.settings',
  ...
);

...and that this saves first_setting in mymodule.settings.json, while second_setting is saved in mymodule.moar.settings.json.

What I need to do though is something like this:

$config = config('mymodule.settings');

$form = array('#config' => 'mymodule.settings');

$form['first_setting'] = array( ... );
$form['second_setting'] = array(
  ...
  '#config_setting' => 'call_this_something_else',
  ...
);

...so both settings will be saved in the same mymodule.settings.json file. The first one as "first_setting", while the second one as "call_this_something_else". So instead of this:

{
    "_config_name": "mymodule.settings",
    "_module": "mymodule",
    "first_setting": 123,
    "second_setting": "abc",
}

...I would instead want to have this:

{
    "_config_name": "mymodule.settings",
    "_module": "mymodule",
    "first_setting": 123,
    "call_this_something_else": "abc",
}