I wanted to add something like this to my wikis. The difference is that the content should be from my wiki. For example, where I have a link to an item (definded by the category, item:<item name>) mouse over it would display some information from that page (it would be nice to include only some information).

I wanted to do this by creating my custom js that would pick up the required data from the source of the page.
The problem wasn't getting it (I used the api from the wikidot js file) but the fact that I can't have any enhancements of this sort on my wiki.