Agreed this is unusual. The airlines could likely send inspection teams with appropriate equipment to the airport they are presently at. It is also probable the FAA would issue "ferry permits" to fly the jets to a maintenance base.

United Airlines and probably some other 777 operators fly cargo-only some flights in those aircraft, since the pandemic restrictions started. This would give operators some flexibility to minimize the number of cancelled flights.

For airlines like United Airlines, they fly both 777-200 and 777-300 aircraft. The United Airlines 777-300 aircraft fly with GE90 engines, which are not grounded.