Quoting "Richard W. Travsky" <rtravsky@uwyo.edu>:
And the first thing that came to mind for me was how DID they shed heat?
Maybe that's what those super-necks and -tails were for; to increase the surface area:volume ratio to help cool down. Feeding adaptations and defense may well have been secondary.
This sounds testable. Increased blood flow, as you suggest below...make a model, pump warm water around, infrared imaging...?
What of the 'spikes' found on some diplodocid necks/tails? Could they have had rich internal blood supplies to further the cooling process (something similar has been suggested for stegosaur plates)? The frill on Amargasaurus also comes to mind.