Roger,
I am positive that you have a lot more experience than me, but I do know this:
If a pump says not to run hot water through it, then it is not meant to have hot water run through it. You cannot fault a General Pump (or Cat, Comet, or Udor *which I had never heard of until a few days ago) for behaving the way it told you it would behave, unless your pump said it was designed to accommodate hot water. Why pump specifically says that hot water will void its warranty. Ceramic, brass, rubber, and steel expand at different rates, which I would think would cause extreme wear on a pump that is not designed for hot water. However, I may be missing something here, so please don't be offended by my post.