Proof of concept for patch.

gistfile1.php

<?php
/**
 * Media class for Soliloquy.
 *
 * @since 1.0.0
 *
 * @package  Soliloquy
 * @author	Thomas Griffin
 */
class Tgmsp_Media {

	/**
	 * Holds a copy of the object for easy reference.
	 *
	 * @since 1.0.0
	 *
	 * @var object
	 */
	private static $instance;

	/**
	 * Constructor. Hooks all interactions to initialize the class.
	 *
	 * @since 1.0.0
	 */
	public function __construct() {
	
		self::$instance = $this;
		
		add_filter( 'media_view_strings', array( $this, 'media_strings' ) );
		add_action( 'print_media_templates', array( $this, 'print_media_templates' ) );
		add_filter( 'wp_prepare_attachment_for_js', array( $this, 'prepare_slides' ), 10, 3 );
		add_action( 'wp_ajax_save_attachment', array( $this, 'save_attachment_fields' ), 10, 2 );
	
	}
	
	/**
	 * Modify WordPress media workflow strings to fit Soliloquy.
	 *
	 * @since 1.5.0
	 *
	 * @param array $strings Default strings
	 * @return array $strings Amended media strings
	 */
	public function media_strings( $strings ) {
	
		if ( ! Tgmsp::is_soliloquy_add_edit_screen() )
			return $strings;
		
		// Modify existing strings.
		$strings['insertMediaTitle'] 	= Tgmsp_Strings::get_instance()->strings['media_title'];
		$strings['insertIntoPost']		= Tgmsp_Strings::get_instance()->strings['media_insert'];
		$strings['uploadedToThisPost']	= Tgmsp_Strings::get_instance()->strings['media_uploaded'];
		$strings['uploadFilesTitle']	= Tgmsp_Strings::get_instance()->strings['media_upload'];
		
		// Remove unnecessary existing strings.
		unset( $strings['createGalleryTitle'] );
		unset( $strings['insertFromUrlTitle'] );
		
		// Return the modified strings.
		return $strings;
		
	}
	
	/**
	 * Output the custom media workflow templates for our custom slide types.
	 *
	 * @since 1.5.0
	 */
	public function print_media_templates() {
	
		if ( ! Tgmsp::is_soliloquy_add_edit_screen() )
			return;
		
		// Custom template for attachment fields. ?>
		<script type="text/html" id="tmpl-soliloquy-attachment-fields">
			<label class="setting" data-setting="soliloquy_link">
				<span><?php echo Tgmsp_Strings::get_instance()->strings['image_link']; ?></span>
				<input type="text" value="{{ data.soliloquy_link }}" />
			</label>
		</script>
		<?php
		
	}
	
	/**
	 * Filter the attachment response to include custom attachment metadata fields
	 * for Soliloquy slides.
	 *
	 * @since 1.5.0
	 *
	 * @param array $response Array of response data about the attachment
	 * @param object $$attachment The attachment object
	 * @param array $meta Array of attachment metadata
	 * @return array $response Amended $response array with our custom data
	 */
	public function prepare_slides( $response, $attachment, $meta ) {
		
		// Return early if not on a Soliloquy post type.
		$parent = get_post( $attachment->post_parent );
		if ( $parent )
			if ( 'soliloquy' !== $parent->post_type )
				return $response;
				
		// Add our custom metadata fields to the response array.
		$response['soliloquy_link'] = get_post_meta( $attachment->ID, '_soliloquy_image_link', true );
		
		// Return our amended response array.
		return $response;
		
	}
	
	/**
	 * Saves custom attachment metadata when WordPress runs the update via Backbone.
	 *
	 * @since 1.5.0
	 *
	 * @param array $attachment Array of attachment data
	 * @param array $changes Changes that have been sent from the server
	 */
	public function save_attachment_fields( $attachment, $changes ) {
		
		if ( ! empty( $changes['soliloquy_link'] ) )
			update_post_meta( $attachment['ID'], '_soliloquy_image_link', esc_url( $changes['soliloquy_link'] ) );
		
	}
	
	/**
	 * Getter method for retrieving the object instance.
	 *
	 * @since 1.0.0
	 */
	public static function get_instance() {
	
		return self::$instance;
	
	}

}