I have a similar (same?) requirement -- I need to show only the media library images belonging to each subscriber.
The logical way to do this would be to add "and post_author = ".$userdata->ID to the wpdb query, but I have been unable to discover how/where to do that.
I was able to unset from the posts array the non-relevant images (wp-admin/includes/media.php, ~ line 1138), but even though the posts array in $wp_query is now reduced to only the images belonging to that user, ALL the images still show up in the display.
I would be extremely grateful for a pointer to the "right way" to accomplish this seemingly simple hack.