CKE4.1.3: configure extraAllowedContent on instanceLoaded

I am using the CKE as an addon for a CMS. The plugins to be loaded are configurable.

The oembed-plugin needs some extra configuration, so I tried to add it on instanceLoaded (after CKEDITOR.replace(...config here...)):

    CKEDITOR.once( 'instanceLoaded', function( evt ) {
        var editor = evt.editor;
        if ( CKEDITOR.plugins.loaded['oembed'] ) {
            editor.config.extraAllowedContent = 'iframe[*]';
        }
    });

But unfortunately, this doesn't work. The IFrame is still stripped from the content.

It works if I add it directly to the replace(...), so I think it's a timing problem. (?) Can anyone help?

I need a programmatical solution 'cause this may affect other plugins, too.

Thank you!