Stem level control across the whole track

With the Stem Separation functionality, I'd like to be able to individually control the volume of each stem part across the track, rather than just as a transition. So for example I can currently right click a track and add a "high marker" to allow me to vary the level throughout the track - I'd like to do the same but at a stem level being able for example to raise/lower/mute the vocals (or other stems) in places. I feel this level of control is needed for mash-ups.