OK new site idea!
The Athenaeum, Old Town Alexandria (map). Really pretty part of town, close to the water.. Not sure why I didn't have this on my list before, a 5-hour Saturday rental, including the required membership, is $1,350. Looks like we could probably do our ceremony there too, in one of the galleries? ..Something in their wording leads me to wonder if the maximum capacity includes multiple rooms.. I would not want to split up our guests into different rooms, so we'll have to take a look.
Maybe my mother, et al., would like to join us on a site visit there sometime soon? :)