In the U.S. registration numbers 1 through 99 are reserved for FAA aircraft (N-1 is a G-V, N-99 is a BAE 125-800 flight inspection aircraft) so the number of digits problem will only happen if the Administrator flies to Europe on her own horse or with the flight inspection aircraft based at Frankfurt.
N100 is a J-3 Cub in Oklahoma....doubt if G controllers will work him, but you never know.