I prefer my manual switch personally, but its an interesting modification. If I were to do something like that I'd use a real automotive switch that closes at 120 degrees or so, mounted in a coolant line to get an accurate reading of whether its warm. Whatever is added, it should not be possible for it to fail in the on position and turn the glowplugs on, or you've just created a worse problem than the one you fixed.
The past two days its been 100 degrees when I eave work, my truck sits all day in the sun on asphalt, and I still glow it or I'll have a 6-8 second crank before it starts. Of course, I don't think many people have glowplugs get stuck or fail that quick with a stock system anyway, if they use Beru plugs and antiseize.