
This, at least, has already been determined: The Blue Cross medical
coverage for all federal employees is a good model for a future
national system. To get emergency medical care, anyone so insured
must always carry their Blue Cross card. Before entering a hospital,
you must notify Blue Cross, or they will refuse to pay your bills. 
In an emergency, where you must be treated before notifying them, 
you must inform them within 24 hours or (if you are unable to do
so for medical reasons) the hospital must. Failing to do so within
24 hours means they will not cover the hospitalization. In you need
your card to notify them (and without the card, the hospital certainly
wouldn't know they had to.) Therefore, you are required to carry
the card at all times, or do without emergency medical coverage.