I'd imagine you're going to have a hard time with this.
Any solution will probably involve creation of a custom component, extending TextArea. Even then, I'm not sure how you would go about doing this, because you'd need to be able to detect which letter (to determine the word) that the cursor is hovering over. It's probably possible but you'll have to really dig into the source for TextArea and it's parent classes, possibly at a very low level.
It definitely would be cool but it may be a lot of work-- are you sure the functionality you're looking for is worth the effort?