I have found it! After seeing this post on Social Mandarin about a plugin for Firefox, I was on the hunt for a similar plugin for Chrome. Lo-and-behold. Welcome Zhongwen. It works amazingly well. Just install it on Chrome, click on or off and you're set. I tested it on Slow Chinese and this is how it looks like.
Just hover your cursor over the words. It looks at the characters next to it very intelligently, and locates words that have more than one syllable. I'm going to keep this on permanently, 'cause I always stumble the odd characters/word I don't recognise. And BOOM, instant pop-up translation. Extremely nifty. I just hope I don't get lazy now.