re: Pipeline Inspection & Magnetism

The inspection is typically done with ultrasonics (smart pig) which will not detect the BBs on the OD surface. If the ID tags were continuous or integral to the surface (welded or clad) the machine would detect it and you could print the tag with the typical shaded/gradient chart. They would have to be attached in some manner that sound would clearly pass through the surface.

If it is a flaw detection technique such as GUL or UTSW or MFL, it detects disruptions in the waveform as the sound or magnetic wave passes through the steel. Magnets would disrupt it, but I'm not sure it would be identifiable and unique like you seem to want.

I'm not as familiar with Pipeline inspection (more process piping in a facility) but my impression is they use Smart Pig most often, and GUL as another alternative. Both of these allows a high confidence inspection over long piping runs.