Does clamshell mode reduce lifespan due to heat?
I doubt that the heat will damage the screen, but considering how MB and MBP feed the fan, I'd be concerned more of airflow cutoff as lots of the air comes through the keyboard itself. You could look into placing a small magnet over the reed switch area and leave the lid open, it's near the Battery Indicator Light. Be careful though, don't swipe a magnet over your HDD.