Interesting idea. It's certainly possible but not a priority for the core dev team. If you feel like taking a look at the code, search for "fwd" and "back" and you can create new "sfwd" and "sback" ControlObjects as copies of the originals only with slower speeds. Then add new buttons that map to those new COs to the skin you're using. Adding GUI configurability (in the preferences) is the hard part, but not terribly difficult. Hop into our IRC channel for more on that if you get that far.
Also, if you have a MIDI controller with some kind of wheel, that replaces using the mouse to scrub/seek/scratch.
Mixxx Developer - Controller Specialist
Mixxx Windows package maintainer