i don't think RS allows anyone to put tags in their name after the character has been created. i might be wrong, i haven't played in a long time, not to mention explore the website.
I don't think there's a rule against tags, but one would have to start over to have a character with such a tag (you're right that one can't be added after character creation). I only know of one guild that does something along these lines, which is the Sabres, who require (or used to require) members to implement "Sabre" somewhere in their usernames.
I don't think this thread is really applicable for us in MMORPGs, because in some like RS it would be difficult, and in most others they already have a guild system implemented which provides such tags.
It's more for those who play shooters, strategy games, and other similar games where one's handle can easily be changed.