But I agree also it doesn't seem to be part of the setting as I see it like glyphs take a while to form most likely so there is a chance it would appear on rocks and plants who remain at that powerpoint for a certain amount of time.
That's on theory. But we do know it may appear on animal skin. Animals do move the same way as people, is if it appears there, it may apper on people's skin too. But also bone, muscle, it is so that you can't just see it, but you know.
But i would like to say one thing as clear as possible: Anyone who is so excited about such a feature, should forget about it at all. While glyphs don't appear often and the fact we may have a lot of them, is because they were gathered since long time and cumulating since then. No quests and if you will be lucky to see such a mage, don't expec to see his whole skin in glyphs. One glyph, maybe powerfull, but nothing more. Unless that guy was choisen by some god, but then don't expect you being that guy, because most likely you aren't good enough RPer.