Hey Matt-
Awesome that you're coming to DC! We generally avoid doing stuff on the mall and near the monuments just so we don't rile the park police too much. If you want to know what's around, check out the hotspots listing on our local site:
http://sites.google.com/site/dcmetroparkour/That being said, we have played on the walls around Union Station a few times and never run into trouble there.
Good luck, and if you get some free time while you're in DC, post it up on the Mid-Atlantic forum - most f us don't check the DC-only forum.
-A