Hmm, i tried to impelented this but without succes. I also wanted to use vtkCommand.InteractionEvent and filtr it by interaction event, but still here was only 4 interaction event without right click press event 
However, I did something like this:
- I added Observer to iren
- the Observer activates when you right-click.
- the Observer takes the position of the click and checks if it is inside the scalar bar.
- executes xyz if it is.