Well...I've been flying open cockpit almost exclusively for thirty years, and and not as sanguine about avoiding overheating.
A well-designed open cockpit isn't all that drafty, and the slipstream normally won't directly impinge on the panel. And an open-cockpit airplane is probably more likely to be flying on hot days. Plus, of course, the tablet will heat up in the direct sunlight, at least during some parts of the day/some aircraft headings.
There's also the case of the airplane heating up while sitting on the ground. With the sun at the wrong angle, it'll be directly impacting the device. Which, while off, has a dark screen and is more likely to absorb solar heating. Sitting all day at a Fly-In isn't going to help. Though, of course, you would probably dismount the panel for theft prevention anyway. But like I posted earlier, I'm more tending towards a permanent installation.
I'm just a little sensitive about the heat-soak issue. Couple of summers back, my Fly Baby's wings swelled in the heat and sun on the ground enough to cause interference to the ailerons.
http://www.bowersflybaby.com/tech/heat_controls.html
But please, don't take my comments as being negative on the concept. I'd be happy to do it, myself. The only way to really know if one would have a glare or heat problem is to actually install an EFIS in an open-cockpit machine, and I'd love to give it a shot. The great things about Piets and Fly Babies is that the instruments are really secondary; they're not truly vital for flight. If an EFIS craps out in flight, those planes can be easily flown back and landed without gauges.
Ron Wanttaja