Toolbar Buttons and Caption/ImageIndex

  I want a TToolBar's TToolButtons to display the caption only when their
ImageIndex is -1 (no image) and to display the image only when the
ImageIndex is > -1 (an image exists).  It appears that
TToolBar.ShowCaptions doesn't work, since it shows captions for all
buttons when set to True.  The IDE's toolbars work as I describe above.  
Ideally, I need to do the above without modifying the actions associated
with the TToolButttons.