Take a long look a the manageControlTool.rex example.  It does exactly what you want, but with tree-view items.  You will need to study the code to realize it is *not* using the tree-view's built in tool tip support.  The tool tip is created and managed completely by the Rexx code.  You just need to apply the technique in the example to the list box of the combo box.

Many thanks for the pointer, Mark. I will lock myself in a quiet room and go through the code with a microscope :)