Image/Attachment URL

I just did that!
After struggling a while, I came up with a somewhat easy solution. Just added the code below at includes/fields/featured_image.php. But then this is no good if f I’m to update the plugin in the future, so I guess there might be a better place to add the code, like in your theme, or in the theme-customisations-master plugin, but I don’t really know how to make sure it’s loaded on time…
The code just duplicates the featured image field code, but replacing wp_get_attachment_image with wp_get_attachment_image_src:

add_filter( 'qw_fields', 'qw_field_featured_image_url' );
/*
 * Add field to qw_fields
 */
function qw_field_featured_image_url( $fields ) {

	$fields['featured_image_url'] = array(
		'title'            => 'Featured Image URL',
		'description'      => 'The "post_thumbnail" URL of a given row.',
		'output_callback'  => 'qw_theme_featured_image_url',
		'output_arguments' => TRUE,
		'form_callback'    => 'qw_field_featured_image_url_form',
	);

	return $fields;
}
/*
 * Image attachment URL settings Form
 */
function qw_field_featured_image_url_form( $field ) {
	//$image_styles = _qw_get_image_styles();
	$image_styles = get_intermediate_image_sizes();
	?>
	<p>
		<label class="qw-label">Image Display Style:</label>
		<select class='qw-js-title'
		        name='<?php print $field['form_prefix']; ?>[image_display_style]'>
			<?php
			foreach ( $image_styles as $key => $style ) {
				$style_selected = ( $field['values']['image_display_style'] == $style ) ? 'selected="selected"' : '';
				?>
				<option
					value="<?php print $style; ?>" <?php print $style_selected; ?>><?php print $style; ?></option>
			<?php
			}
			?>
		</select>
	</p>
<?php
}

/*
 * Return the images URL
 *
 * @param $post
 * @param $field
 */
function qw_theme_featured_image_url( $post, $field ) {
	$style = $field['image_display_style'];
	if ( has_post_thumbnail( $post->ID ) ) {
		$image_id = get_post_thumbnail_id( $post->ID, $style );
		return wp_get_attachment_image_src( $image_id, $style )[0];
	}
}