My compliments for doing your due diligence John. I'm not aware of any NDT lab with significance experience in evaluating pattern welded barrels, and suspect it may be related to liability concerns.
It is my not particularly informed opinion that direct visualization, RT and MPI are complimentary. If MPI showed an indicator at the same location as the visible "crack" I would be apprehensive.
This is Jay Oliver's 1874 $100 grade Lifter with a fracture in the Crolle barrel probably at the rod edge "zipper" weld
A Twist fracture
Damascus-Twist fracture
Threaded Mode