That's why I say it will depend on how the law is applied. You cannot restrict an individual from possessing the gun and the open display laws are also applied individually, not to a group.
Right now you can restrict carrying ANY gun in public. That has yet to be overturned. For DC, its has so far been upheld by Heller II, which has been decided by the D.C. Circuit and so far SCotUS seems unwilling to take up Heller II to clarify/extend the scope of Heller I/McDonald. Until SCotUS takes up Heller II or other similar gun control cases, DCs current, post Heller, gun control laws are upheld.
And it doesn't matter if the laws goes to individuals. A group is made up of individuals. Each individual carrying would be breaking every currently upheld DC gun control law the moment they stepped foot in DC. That is at minimum 2 felonies per person, 3 or 4 if they are carrying assault weapons with extended magazines.
In DC it is currently illegal to open carry firearms, as well as possess extended magazines, assault rifles, and guns not registered in DC. IIRC each are felonies.
My bet is only the nuttiest of gun nuts participate. Most rational people aren't going to risk getting charged with multiple felonies. This will just heightens the risk involved because you will truly have the crazies participating.
If the March comes to fruition. I see choppers hoovering over the middle of the bridge, likely urging all to turn back before entering DC, while snapping high rez photos of everyone. I see Washington DC and the Feds, boxing off and barricading the end of the bridge on the DC side with uniformed riot cops and tactile vehicles. There would likely be a second line of uniformed officers in back waiting to make arrests. They have the delusions that they will all be allowed to return back to the Virginia side without being arrested. They might, but that doesn't mean they won't be charged. If DC and the Feds wanted to they could wait until the marchers got close to the one end of the bridge and then drop in swat units to the middle of the bridge thus corralling the marchers.
The march would be a high stakes game of chicken. I honestly don't see the march taking place, but since now its says hes working with police to determine what line they cross results in arrest, if it does happen, I doubt they cross over the middle part of the bridge.