I had a quick look into it and unfortunately there seems to be no easy way to accomplish this at the moment. The parts of functionality that you would need here are buried quite deep inside the Slider implementation.
The Slider class is not ideal in many aspects and there is a chance that we will refactor it at some point in the future. Until then - unless anyone else has a better idea - I would suggest that you take a look at the mouseUp/mouseDown implementation in Slider, and at the behaviour for SliderStyle::IncDecButtons (which already does what you want, but unfortunately with extra buttons) - maybe it can be done by combining the code in there?