The hoop stress calc seems to be a way to start, and it may be the way to the end. Let me ramble for a minute to explain.
Any place that the pipe is still pipe-like (i.e., round and intact), was subject to forces equal to the yield point, and that is what that equation shows you. So I guess you could solve it for the pressure using the pre-yield diameter to get the pressure when it probably started to yield (the published number is the minimum specified yield stress, actual yield point is always higher than that, it is not possible to generalize how much higher or to go back to a piece of pipe that has yielded and test it since the act of yielding changes the mechanical properties).
That pressure will be the highest that the pipe saw since the tensile strength is less than the yield strength and the yield point is a maximum. If the pipe stretched to 150% or 500% it doesn't matter since if you redo the equation above for the larger pipe diameter you'll get a lower pressure (i.e., the diameter tends to change a larger percent than the wall thickness change). The new diameter is just a measure of the pipe's elasticity, not a measure of the pressure it was subjected to.
I'm thinking that the hoop stress calcualtion will give you the overpressure, but it will be very conservative and I would present it as an "at least" number.
David