TYPO3 Core

If you add the following code to add a new field to the site-configuration, you can easily test this. Note: The site_base_variant is re-used for a second configuration option. In fact i tried to do exactly that to add UI Support for the CORS Configuration of a Consent-Management Extension i built. I would have added a custom site-configuration data type, but this is not possible, so i tried to re-use the site_base_variant.

If you add this and then create a new site-config, add 2-3 base-variants and 2-3 items to the new column you can see, that they will override each other.

$GLOBALS['SiteConfiguration']['site']['columns']['example_column'] = [
    'label' => 'Example',
    'config' => [
        'type' => 'inline',
        'foreign_table' => 'site_base_variant',
        'appearance' => [
            'enabledControls' => [
                'info' => false,
            ]
        ],
        'overrideChildTca' => [
            'columns' => [
                'base' => [
                    'label' => 'Override',
                ],
                'condition' => [
                    'config' => [
                        'eval' => 'trim',
                    ],
                ],
            ],
            'types' => [
                '1' => [
                    'showitem' => 'base,title,condition',
                ],
            ],
        ],
    ],
];
$GLOBALS['SiteConfiguration']['site']['types']['0']['showitem'] .= ',example_column,';

The problem is, that the form data will be named using the record type insteadof the property name

in pseudocode:

It should be:

// form data for baseVariants
site[baseVariants][0][base] = "foo";
// form data for custom field
site[example_column][0][base] = "bar";

But it is:

// form data for baseVariants
site[site_base_variant][0][base] = "foo";
// form data for custom field
site[site_base_variant][0][base] = "bar";

You can still add all the config you need to the site-configuration but you cant make a GUI for it.... No custom data types, no re-using of existing data types.