In perpetuity in a court of law means exactly what it means.
Do the Cubans have a right to fire on the United States installation at Guantanamo bay because the lease for the facility was signed by a previous Government.
It means the same thing when referring to Allegiance to King George III.
"Allegiance is founded in the Relation every Man standeth in to the Crown considered as the Head of that Society whereof He is born a Member; and on the peculiar Privileges He deriveth from that Relation, which are with great Propriety called his Birthright. This Birthright nothing but his own Demerit can deprive Him of; it is Indefeasible and perpetual. And consequently the Duty of Allegiance which ariseth out of it, and is inseperably Connected with it, is in Consideration of Law likewise Unalienable and Perpetual.