Mockup for the Mixer Navigation bar idea:
Click on the image to get the full size image:
In that image you'll see a 75 track session. On a 1920x1200 monitor that leaves at 20 pixels per track for the navigation display. Even at 300 tracks, you'll be able to have a reasonably overview. The problem may arise when the user has the mixer window resized to something so small that only a few pixels per track are possible. In that case it may be necessary to eliminate the 1-pixel divider space and just plot the track colours.
Another problem is that the theme images in the label area and the track colours mix to produce a new colour. It may be necessary to sample the actual resulting image colour to generate a more accurate 'thumbnail' colour for the mixer navigation bar.