Hmm... I don't know of anything for IE myself, and remember the plugin architecture being a big mess when I looked at it, so it wouldn't surprise me if there isn't anything.

That said, there is (or was) a piece of software years ago called "Dr. Eye" that was published by a Taiwanese company and did popups at the level of the operating system in Windows, so it should be usable with Internet Explorer. I am not sure if it is still around, but it was a commercial application instead of a free dictionary tool.