YOU CAN USE MAGNETS!!!! - I did...
Just use strong enough magnets and make sure that they are installed correctly for maximum strength. I had weapons attached using magnets all through DCon and didn't have any issues. I still believe that the correct magnets will come out on top everytime when compared with velcro or something similar. You have to be able to remove the weapon using some amount of force, otherwise what's the point? If you can pull it off it doesn't matter if it's a magnet, velcro, or unobtanium...
I would agree that engineering a system capable of holding a full sized AR throughout a Con would be quite a challenge - maybe for next year...