Sorry @ike,
We switched to a new server during the weekend, your post (and my answer) must have been removed.
I’m adding a new filter for you, includes/functions.php line 435 was:
$filepath = $uploads['basedir'].'/'.$attachment;
will be:
$filepath = $uploads['basedir'].'/'.$attachment;
$filepath = apply_filters('da_download_attachment_filepath', $filepath, $attachment_id);
You have filepath and attachment ID, you can apply any conditional statements here. Use it – it will be there after plugin update.
But you can also filter file visibility in da_get_download_attachments() and da_display_download_attachments() functions – there are filters avaiable already. The one we talk about above is for direct download – when someone knows the url or tries to get download url to process file download. It’s not responsible for links display.