Just with an idea, that i think willl be very helpful, but don't know if is dificult.
The last versions of X server come with the capacity of using two pointers. (Details: http://alec.mooo.com/mpx.html ). Given the fact that most laptops come with a trackpad, and is easy to conect a usb mouse, most users have the capacity to have two pointers. Although it doesn't has a lots of uses, i belive that could be useful to control the Crossfader with the trackpad, while keeping control of the computer with the mouse (or the other way around). Giving the user a "Control surface", making easier to mix.
For having this capability the only thing needed is the ability of capturing the pointer (one of them) with the crossfader.
Hoping this be posible, and be helping.