I believe the ToolTip class is complete, so there should be no reason why you can not create and manage your own tool tip for a combo box.

In one of the examples for tool tips, I created and managed my own tool tip completely independent of the list-view.  That is, it does not use the list-view's tool tip.  One of the intents their was to sure that it is possible to do any thing you want with a tool tip.  Just takes some work.  ;-)

That sounds good enough for me, thanks!