I was just doing this the other day. I tried creating my own hovering tooltip using Dialogs, but there is no /did for resizing text controls, nor for detecting font information to determine correct $width(). The best I can come up with is an always-on-top desktop tool window that is dynamically positioned nearby the $mouse.x and $mouse.y, or in static relationship to $window($active).dx .dy and .dw .dh (two different coordinate spaces, i know).
There are no very pretty solutions. A /tooltip command would be a nice compliment to balloon /tips.
Oh, and I'm using On Hotlink for hovering over nicknames in channel, but not the nicklist.