Hello Dave, as I read your post it reminded me of a condition that is very prevalent in many of the mills and other manufacturing facilities in my area and likely in many other's too. A large share of "mill or manufacturing facility" maintenance welding machines use a ground that is connected to the facility's structure and then they use a long stinger lead to perform the work wherever it is needed, or possibly have several stinger leads going to different locations on the plant site from a common machine, this avoids having to run two lengths of welding cable(one for the stinger and one for the ground). Using the structure of the building for the ground is likely also making the machine ground common to the various power panels that are grounded to the framework of the building. Thus, you can certainly end up with current taking paths that you don't want it to. This is also likely why the use of jumper grounds is strongly recommended when this type of condition exists, although it still doesn't guarantee the correct current path will be taken.
I believe this same scenario can be applied to structural steel erection and welding as many welding rigs will ground near to the machine through the structure and then run the stinger lead to wherever it is required to make the necessary welds. When I cover welding machine safety for my students I include a section on proper grounding and discuss the use of common grounding and some of it's drawbacks. One of the scenarios that I cover with the students follows somewhat the line that you discussed in your post about the arcing in the rivet yoke that you mentioned. The example that I give includes using the structure's ground and welding elsewhere in the building, possible arcing could occur due to a similar condition as you described, only in this case maybe the arcing is taking place in the chemical storage area of the building, this could possibly cause a fire to erupt without it being obvious to those working. This is highly unlikely, but the possibility still exist. Lots of hypotheticals, I know, so please forgive me. Best regards, Allan