Didn't we have a thread on this recently?
Officially, outta luc. But on a weekend, and not a regular thing, I'd think you'd be ok.
http://www.wmata.com/rail/parking/
Multi-day parking (also known as overnight or long-term 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 of up to 10 days. Availability is on a first-come, first-served basis. There is no charge for multi-day parking beyond the regular fee. See the individual station pages for more details about multi-day parking at these three stations: