I just got off the phone with Mac Wyckoff, who is a Historian with the Fredericksburg - Spotsylvania office of the National Park Service. I reached him at (540) 373-6122.
Now we all know how much veracity there is in anything that is not printed on letterhead stationery and not notarized, but notwithstanding -
Mr. Wyckoff says that the boundary of Wilderness Battlefield extends to the shoulder of the road along Virginia State Route 20 (The Constitution Highway/The Constitution Route).
If you want to trust your liberty and future to my recounting of Mr. Wyckoff's statement, You are "good to go" as long as you do not pull off the paved road surface when passingthrough there.
I think I will take on a project to get info on the borders of all of the NPS land in Virginia. Not sure how it is recorded, so I'm not sure how I could post it. But it just might keep me too busy to loom for a while.