sounds like a cool idea.
If you are adding rollovers using onmouseover events on words in JCross clues, then I don't think there will be a conflict with the JavaScript that Hot Potatoes and QuizPort use.
BTW, did you know that Moodle already has the overLib library available, "lib/overlib.js "? What's more, if you are using a standard Moodle theme then "lib/overlib.js" is included automatically for you
While developing, I suggest you get the JCross working outside of Moodle first, then try it with QuizPort.
all the best
Gordon