There was a militia unit known as the Morgan Continental Guard that wore such uniforms. Based in Winchester, VA, they were mustered in as Co. K of the 5th Virginia Infantry, which would have put them under Jackson's command at 1st Manassas. I heard that a reenactors group representing that particular unit did participate in the filming of Gods and Generals, which makes it very likely that they were the soldiers you saw on the screen.
The presence of so many militia units dressed in all different styles and colors of uniforms led to plenty of confusion at the battle. I remember visiting the Manassas battlefield years ago and hearing a park ranger tell the story about a particular cannon on Henry Hill.
The story goes that a Union officer came upon a gun crew preparing to fire at some blue-uniformed soldiers across the field. He ordered them to hold their fire.
The leader of the gun crew remonstrated with the officer, saying that he was quite sure that the intended targets were Confederates. But the officer insisted that, if they were wearing blue uniforms, they must be Union, and repeated his order for the crew to hold their fire.
The crew obeyed the order and refrained from blasting away at the blue-uniformed soldiers. Those soldiers, who were, indeed, Confederates, moved in and captured the gun moments later.