Extend WP_Customize_Image_Control class allowing access to files uploaded within the same context.

wp-customize-image-reloaded.php

<?php 
/**
 * Customize Image Reloaded Class
 *
 * Extend WP_Customize_Image_Control allowing access to uploads made within
 * the same context
 */
class My_Customize_Image_Reloaded_Control extends WP_Customize_Image_Control {
	/**
	 * Constructor.
	 *
	 * @since 3.4.0
	 * @uses WP_Customize_Image_Control::__construct()
	 *
	 * @param WP_Customize_Manager $manager
	 */
	public function __construct( $manager, $id, $args = array() ) {
		parent::__construct( $manager, $id, $args );
	}
	
	/**
	 * Search for images within the defined context
	 */
	public function tab_uploaded() {
		$my_context_uploads = get_posts( array(
		    'post_type'  => 'attachment',
		    'meta_key'   => '_wp_attachment_context',
		    'meta_value' => $this->context,
		    'orderby'    => 'post_date',
		    'nopaging'   => true,
		) );
		?>
	
		<div class="uploaded-target"></div>
		
		<?php
		if ( empty( $my_context_uploads ) )
		    return;
		
		foreach ( (array) $my_context_uploads as $my_context_upload ) {
		    $this->print_tab_image( esc_url_raw( $my_context_upload->guid ) );
		}
	}
}

/**
 * Example of inserting a section called "Branding" with a
 * context-based image uploader
 */
$wp_customize->add_section( 'my_branding', array(
	'title'    => __( 'Branding', '' ),
	'priority' => 30,
) );

$wp_customize->add_setting( 'my_logo', array(
	'capability'  => 'edit_theme_options'
) );

$wp_customize->add_control( new My_Customize_Image_Reloaded_Control( $wp_customize, 'my_logo', array(
	'label'   	=> __( 'Logo', '' ),
	'section'	=> 'my_branding',
	'settings' 	=> 'my_logo',
	'context'	=> 'my-custom-logo'
) ) );
?>