Hi Frank, you mentioned that it took the pyrex about ten minutes to be damaged, but I'm certain if you had water flowing through it and drawing the heat away, it would have lasted.
XWZCIENCE has done some work with steam tube boilers, and one of his posts is about a coil he made from very thick-walled steel pipe. He indicated the mass of the steel acted as a thermal buffer that absorbed the fire heat and evened out the heat transfer to the water-steam conversion, making it steadier and more controllable.
If anyone still asks about you shooting planes out of the sky... (I agree with Volvo Farmer) If the focal point is 12' out, then the "hot zone" is everything within 24', and will look like two funnels connected at their tips (similar to an hourglass shape). The reflected light (to a plane or neighbor on an elevated hillside) will be no worse than any 1-foot flat mirror.
If you don't want to move the entire dish if flow stops from a failure, you might also consider a system where the heat-exchanger moves just a couple feet. Such a system might also allow you to adjust the position of the HE for maximum absorbtion on a mild day and partial absorbtion on a sunny day.
I have seen a hydraulic system on aircraft where the actuator moved as it cycled the flaps. Rather than supply it with a short section of rubber hose, a short section of metal tube was used for longevity. It had been coiled with 5 turns, and the coil was aligned with the actuators base pin. I expressed suprise, and my boss looked up the specs, then assured me 5 coils of stainless steel tubing could take millions of cycles. The pic below is from a moonshi...um...I mean a "water distiller".
http://www.conquestinc.com/images/Life%20Saver/LifeSaver4.jpg