Media module function overriting, user can see only own media
The best suggestion I can come up with is to use hook_form_alter() to alter the value of $form['admin']['#files'] (inspect the form array on the /admin/content/media page) by calling your own version of media_admin_list() with an altered query.

Something like:

function MY_MODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if($form_id == 'media_admin') {
        $form['admin']['#files'] = custom_media_admin_list();
    }
}


This is slightly inefficient as the original function has already run but I can't see a way to alter it before the initial form array has been built.