Sorry, I looked into it and it doesn't seem feasible to add. The biggest issue is that I don't see a way of searching for other users on GameCenter and viewing their profile without having them added as a friend.
I did see Apple is coming out with a new Arcade service. Maybe they will change some things when that arrives.
edit: I moved discussion to a new thread so it's not in the Battle.net one.
Yeah, unfortunately in iOS 12 (I think, can't remember the version) Apple removed Gane Center app, only leaves a backbone in settings and that's about it.
Hopefully Apple Arcade will change that.
I was about to ask the same thing lol