This may have slipped by most people, but there is now an option in your Profile to specify which platform you play on.
Simply click your name, then User Control Panel, choose the 'Profile' tab, and you'll see the option right there under the box where you enter your Commander name.
Your platform will then be shown under your Commander name, below your profile pic on all your posts.
Thanks to Edddelos for the suggestion, and to Saberius for doing the clever stuff under the hood.