The District recently secured federal funds to replace the Long Bridge Railroad bridge over the Potomac. If the bridge is to be replaced, and the $2.9 million secured is probably only enough for a study, then it would be an excellent opportunity to add the new non-motorized bridge called for in the 14th Street Bridge EIS.
The Long Bridge is only about 1000 feet downstream from the George Mason Bridge and it's ATL, but the idea of a bike bridge south of the Long Bridge is already being considered, and this would be a cheaper way to do it (albeit closer to the Mason than the one in the 14th Street EIS). And there is a bridge planned to connect the Mt. Vernon Trail to Long Bridge Park between Roaches Run and I-395. Perhaps it would be wise to add the bike bridge to this project. If so it would make sense to allow cyclists and pedestrians to continue across the river on the same line, rather than make them drop down to the Mt Vernon Trail, and then move up or downstream to another bridge. An ATL along the rail bridge would give a more direct connection to Ohio Drive and it could even be carried further, across East Potomac Park and channel to Maine Avenue and 14th Street. If you wanted to get really crazy, the rail-with-trail could continue all the way to the park south of C between 7th and 9th. There is space along the rail corridor and even an extra tunnel under Maryland Ave SW, of course the land belongs to CSX so that would be a tough negotiation to say the least (and it may be the ROW is better used for high speed rail).
I'm not sure it would be worth the money (as I have no idea how much it would cost), but it is something that should be looked at. DC is in a flurry of bridge rebuilding right now, and it's a great opportunity for cyclists.