Bernhard, good to hear from you.
The pole load was built following an article in the February 1992 issue of Mainline Modeler, by Bill Darnaby. There's an accompanying table showing AAR loading instructions and specifications, including those for an overhanging load. You're correct in stating that most loads should be placed so that the weight is centered about the car's wheelbase, but I suspect the pole load nowhere near approaches the car's 70 ton capacity. The excess weight at one end would be negligible. The only reference in the table to overly-long loads is that they be supported on an 8"x8" timber, placed at the end of the carrying car where the load overhangs - this is to ensure that the load
does not rest at all on the idler flat car. I'd guess that heavier over-length loads, or longer over-length loads would need to be centered in the supporting car, with an idler at both ends.
While looking for that article, I found another interesting one on open-car loads, in the June 1995 issue of RMC. While there's nothing on extra long loads, there's plenty of interesting stuff (with model photos) and all braced/blocked following information gleaned from the 1950 edition of the AAR's "Rules Governing the Loading of Commodities on Open Top Cars".
(And I was right about my pipeloads - they're stacked too high.)
Wayne