I'd let a dragon fly wearing custom-built armor, even if it were full plate, providing, of course, that the dragon is proficient with the armor being worn.
In another RotG article (I think it was RotG), one of the carrying capacity ones, it was mentioned that dragons count as quadrupeds while flying with regards to carrying capacity, because dragons are seen as being incredibly strong, so, using that arguement, it would be permissible to have a dragon flying in heavy armor. The carrying capacity is sufficiently large that it'd be a light load for most dragons (exceptions being extremely young dragons).
