Yeah, it's possible. Just a few weeks ago or so someone linked a photo where a fat had collarbones. Unluckily for her she had a mirror behind her, so you could spot the weird fucking angle she had her shoulder/head in.  Also I think if you push your shoulders forward på shoulders very far back and use MySpace angle it might give an illusion of collarbones. (just tried it myself. Not sure about the successrate if fats use it. All I know is that I look fucking retarded while doing that).