Media uploader & manager: add/remove class on image with checkbox

Question

This seems like it should be fairly simple, but I'm drawing a blank on it:

Have a checkbox in the media uploader and media manager that, when checked, adds a CSS class name to the image. It should stay checked.

The kicker is I can't use post meta to store the checked or not checked status. It's a long story, but basically the requirements of the project I'm working on say that setting a global meta key/value on the image is not OK.

I figured out how to add the checkbox and set/unset post meta with it:

/**
 * Add a checkbox to media uploader & manager to mark an image as a Profile Image
 *
 */

function my_select_profile_image( $form_fields, $post ) {
   $is_profile_image = (bool) get_post_meta($post->ID, '_is_profile_image', true);
   $checked = ($is_profile_image) ? 'checked' : '';

    $form_fields['is_profile_image'] = array(
    'label' => 'Profile Image?',
    'input' => 'html',
    'html' => "<input type='checkbox' name='attachments[{$post->ID}][is_profile_image]' id='attachments[{$post->ID}][is_profile_image]' {$checked} />",
   'value' => $is_profile_image,
    'helps' => 'Checking this box will add profile image styling to the image & caption.'
    );
return $form_fields;
}
add_filter( 'attachment_fields_to_edit', 'my_select_profile_image', null, 2 );

/**
 * Update image meta based on Profile Image checkbox status
 *
 *
 */
function my_save_profile_image($post, $attachment) {
    if( isset( $attachment['is_profile_image'] ) )
        update_post_meta( $post['ID'], '_is_profile_image', true );

    if( !isset( $attachment['is_profile_image'] ) )
        update_post_meta( $post['ID'], '_is_profile_image', false );
    return $post;
}
add_filter( 'attachment_fields_to_save', 'my_save_profile_image', null, 2 )

But with post meta off the table, I'm at a loss on how to do this.

Answer 1

Given this

I can't use post meta

I suppose this is a typo (my emphasis)

setting a global meta key/value on the image is not OK

It's either a post meta or a site option.
To use it as site option, use this in attachments_fields_to_edit:

$is_profile_image = get_option( '_is_profile_image' );
$checked = isset( $is_profile_image[ $post->ID ] ) ? 'checked' : '';

And this on attachment_fields_to_save:

function my_save_profile_image($post, $attachment) 
{
    $is_profile_image = get_option( '_is_profile_image' );
    if( isset( $attachment['is_profile_image'] ) )
        $is_profile_image[ $post['ID'] ] = true;
    else 
        unset( $is_profile_image[ $post['ID'] ] );

    update_option( '_is_profile_image', $is_profile_image );
    return $post;
}

The site option _is_profile_image will be an array like:

array (size=2)
  96 => boolean true
  97 => boolean true