Is it possible to link a slider controller to the outgoing/incoming velocity, these values are visible when you double click a keyframe in the graph editor where you are able to manually enter a set value (as per screen shot) I was wondering if I was able to control the highlighted fields via a slide controller?
No and I don't think one should do it. That property is not suppose to be animated since it is what determines the animation. Giving access to it would break things because you could link more than one keyframe to the same controller. Setting counteracting values.