Took 3 weeks, but here is the "official" response:
Multiple day parking is available at three stations: Greenbelt, Huntington and Franconia-Springfield. Each of these stations has between 15 and 17 spaces allocated for multi-day use, which are available on a first-come, first-served basis. There are signs indicating which spaces are reserved for multi-day parking.
Please let us know if we can be of further assistance.
Sincerely,
Office of Customer Relations