This should let you know what the DC & area properties offer on the weekend (most seem to offer full brekkie or buffet brekkie). It's not totally updated, but gives you a good overview.

http://www.flyertalk.com/forum/13849211-post1.html

Marriott's T&Cs is that if an exec lounge is closed or there isn't one, elite+1 get cont'l brekkie. Often properties will either do full breakfast (off the menu) or full buffet breakfast even if the policy is cont'l.

If buffet breakfast only is important to the OP, then he should probably contact the properties before staying at them to find out what they do when the lounge is closed.

Cheers.