Now, the question still stands, is it ok to put a piano on an
exterior wall???
I have done so for years. In fact, I can think of 4 different pianos that I have owned, and all of them spent nearly their entire lives (in my possession) on an outside wall.
For logistical reasons, I have not been able to place them far away from heat sources, but I have been able to provide humidity to their environment. The only problem that I ever had was some fading to the walnut finish of a 1926 piano that received some direct sunlight.