If nothing else, they'd be too busy laughing to do anything invasive. You are correct, though, each "layer" of medical care provider has different legal obligations and must be notified in the proper manner. If one wants a DNR order, fine, but that has to be done properly and everyone, from paramedic to ED to floor, must have their own tees crossed and eyes dotted. Consult with a professional estate planner or lawyer in your area who is familiar not only with the State laws but the local emergency medical services' policies and procedures as well.
Important note to families: Do not get too upset if heroic measures are being taken; the staff (ambulance, ED, whatever) may just not have been properly informed. Remember that the default procedure is to use all heroic measures.
JD86, your client's idea is pretty clever. He should be sure to have a copy of the form on his person, were he to collapse outside his home.