Not really. It's perfectly common as addressing a singular person of unspecified gender, and I can't think of a common way to use it to specifically reference royalty offhand.
I guess the plural in some languages is used to address royalty. And about the usage of "they", I tend to use it, and the only thing that I find a bit strange is sentences like: "If a user needs something, they have to do something." Note "needs" and "have" for the same subject, where usually they are either needs+has or need+have.
Yes in German the plural is used for royalty. I'll try to adapt to "they" but I have to fight a strong sense of wrongness that I have acquired somehow.
