OfiZIHalmente Ruco

Don't think it works that way. Here's the start of the section about foreign coverage. The last paragraph is the key where it says you must live in the USA. So while you can sign up if your a citizen, if you don't live in the USA you'd have no coverage. You don't receive medicare when incarcerated, but it doest affect your eligibility.

Medicare usually doesn’t cover health care while you’re traveling outside the U.S. There are some exceptions, including some cases where Medicare Part B (Medical Insurance) may pay for services that you get on board a ship within the territorial waters adjoining the land areas of the U.S. However, Medicare won't pay for health care services you get when a ship is more than 6 hours away from a U.S. port.

Medicare may pay for inpatient hospital, doctor, and ambulance services you get in a foreign country in these rare cases:

You're in the U.S. when a medical emergency occurs, and the foreign hospital is closer than the nearest U.S. hospital that can treat your medical condition.
You're traveling through Canada without unreasonable delay by the most direct route between Alaska and another U.S state when a medical emergency occurs, and the Canadian hospital is closer than the nearest U.S. hospital that can treat the emergency.
You live in the U.S. and the foreign hospital is closer to your home than the nearest U.S. hospital that can treat your medical condition, regardless of whether an emergency exists.
Medicare may cover medically necessary ambulance transportation to a foreign hospital only with admission for medically necessary covered inpatient hospital services.