What would admissions at competitive universities (MIT/UPenn/Cornell/Vanderbilt/… think of this?

If you explain what's involved in the programming (hopefully actual programming that leads to skills in other languages) then it can be a plus to your application. The fact that it's for WoW won't hurt since you didn't say you spent all your time on it. But don't count on it being a huge help to your application, more of a curiosity.