I do note that if you include a custom dictionary, you can get popup definitions in your book, which I think is going to be similar to what you want, but it's non-trivial to create, and I don't know of a wysiwyg editor which creates them...
see
http://www.mobipocket.com/dev/articl...e=indexing.htm
and here:
http://www.mobipocket.com/dev/articl...eFolder=prcgen