Theme Customiser Image Control

The solution, needs a custom control object extending the original image control, and does an SQL query to grab the GUID and associated attachment ID on sanitisation. Not nice, kludgey, but it works

$wp_customize->add_setting( 'customimage', array(
    'default'       => $default,
    'capability'    => 'edit_theme_options',
    'type'          => 'option',
    'sanitize_callback' => array( 'ICIT_Customize_Image_Control_AttID', 'attachment_guid_to_id' ),
    'sanitize_js_callback' => array( 'ICIT_Customize_Image_Control_AttID', 'attachment_guid_to_id' ),
) );

$wp_customize->add_control( new ICIT_Customize_Image_Control_AttID( $wp_customize, "custom_image_attach_id", array(
    'label'      => $label,
    'section'    => "custom_image_attach_id",
    'settings'   => 'customimage'
) ) );

if ( ! class_exists( 'ICIT_Customize_Image_Control_AttID' ) ) {
    class ICIT_Customize_Image_Control_AttID extends WP_Customize_Image_Control {

        public $context="custom_image";

        public function __construct( $manager, $id, $args ) {
            $this->get_url = array( $this, 'get_img_url' );
            parent::__construct( $manager, $id, $args );
        }

        // As our default save deals with attachment ids not urls we needs this.
        public function get_img_url( $attachment_id = 0 ) {
            if ( is_numeric( $attachment_id ) && wp_attachment_is_image( $attachment_id ) )
                list( $image, $x, $y ) = wp_get_attachment_image_src( $attachment_id );

            return ! empty( $image ) ? $image : $attachment_id;
        }


        public function attachment_guid_to_id( $value ) {
            global $wpdb;
            if ( ! is_numeric( $value ) ) {
                $attachment_id = $wpdb->get_var( $wpdb->prepare( "SELECT ID FROM $wpdb->posts WHERE post_type="attachment" AND guid = %s ORDER BY post_date DESC LIMIT 1;", $value ) );
                if ( ! is_wp_error( $attachment_id ) && wp_attachment_is_image( $attachment_id ) )
                    $value = $attachment_id;
            }

            return $value;
        }
    }
}