I did some tests and unfortunatelly air combat and ranged combat are completly different. That is the reason no xp is granted from ranged combat. I had to workaround this by giving siege engines free xp, just as mages. By the way, is it possible to change this?
Anyway, I tried both improvements and buildings. In fact buildings already have the same tag as improvements for air combat, it is just called <iAirModifier> and that is why I did not found it at first.
Anyway, here are the results. Air damage protection from both improvements and buildings do not affect inflicted ranged damage at all. Bombard protection from buildings does not work, either.
However, normal city and improvements defences do work. Even normal defence from forests and hills works

I did not spot it in the normal play as the effect is not big. Unprotected unit got 21% damage in one of the tests, while protected in the city (210% defence) got 14%. Also, the protection seems to have diminishing results as it goes up, as I got 16% with just 75% defence.
So, no need for a new tag, but if the damage protection could be improved, like 50% protection at 100% defence, it would be great. I think that lowering ranged damage from 21 to 11% at 100% defence would make more sense.
By the way, I discovered that units that ignore building defences ignore their protection from ranged attacks too. And that is exactly how I wanted it to work...
