A DND should be a DND, and nothing more than a notecard under the door or voicemail message.

But having worked in Housekeeping years ago, I know from personal experience how many guests leave the DND sign on the door and have a fit when they come back and their room isn't cleaned because some how we were supposed to know that they didn't really mean to leave the DND there. So I can't blame some hotels for trying to get in there.