True Scott, if you could keep the temp at 160F, like the graph shows for the 5800 series http://www.delavanagpumps.com/downloads/PowerFLO/PF 5800-5900 Install.pdf, you should be fine for 2 hours or so.
You could also wire the pump with some kind of thermal switch. Or run two pump intermittently off the generator for the big projects.
Good ideas, but I think its getting a little too fancy, when just opening and closing the ball valve completely does the trick. We've been using the same Fatboy w/ the same relay switch since August or September, w/ no issues. Its actually the longest any pump has lasted for us and we figured our luck would run out soon, so we ordered another one as a backup just in case. lol.