Icing Tests
Maybe one reason for not testing the pitot for icing at high altitude is the lack of an objective test. Airframe, and presumably pitot, icing testing is performed at lower altitudes by following a plane that sprays droplets.
Maybe now with the availability of DC-10 and 747 aerial firefighting tankers, realistic, objective icing tests at cruise altitudes could be performed.
Takata, what is the source of your info that pitot icing in light General Aviation "is one of the main accident cause."?