Lots of great ideas here, thanks folks. I'm sort of leaning toward a Pontiac Vibe type of solution. Although the dog lifespan is only another ~4 yrs, that's still another 4 yrs that we should try to take care of the dog. Tinting the windows is definitely on the radar, though I know its only slightly helpful. Fans don't help dogs very much, they real only cool down via convection on their noses, paws, and belly; so a small aux fan in the back would only be a small help. Still, perhaps a small fan, tinted windows, and a spillproof bowl (ice) is the most reasonable solution. Our drives are often the annoying 2 mile variety (steep-ass hills with kids that don't always want to bike in bad weather on narrow streets. We bike roughly 60% of our commuting miles), but we also do 300 mile trips every other month, and then a pretty big western road trip in the summer. Just barely enough for me to seriously take MPG into account over the general posh nature of a minivan. When you take the average of 21mpg of a Honda Odyssey vs to the 29 mpg of the Pontiac Vibe into account, I'm going to lean toward the improved fuel efficiency and slight loss of comfort.
Those old Roadmasters sure look sweet.