Thanks Sumedh for the quick & definitive response.
A related question perhaps to make a work around - can we trigger JavaScript events on outliers (clicking, mouseover, ...)?
I found http://docs.fusioncharts.com/charts/?AttDesc/ToolTip.html and have gotten custom tooltext to work for regular graph plotting (in my case a Box Chart), but I can find no documentation for adding custom tooltext to outliers. Is this possible?