Okay I think I have it, you need to add a matching filter to attachment_fields_to_save
to save the new field (say as post meta to the attachment.)
add_filter( 'attachment_fields_to_save', 'save_some_custom_field', 10, 2 );
function save_some_custom_field($post, $attachment) {
$attachid = $post['ID']; // yes this is actually an array here
update_post_meta($attachid,'someCustom',$attachment['someCustom']);
}
Then you would add to your $form_fields
edit filter array something like:
'value' => get_post_meta($post->ID,'someCustom',true),
…remembering to remove the = null
from the function arguments.
unset($form_fields['post_excerpt']);
and unset($form_fields['post_content']);
in the edit filter should remove caption and description fields respectively, not sure on the required part though.