I was thinking that you'd just have full transparency when thr = 0, full opacity when |thr| >= thr_slider, and linear scaling in between, since the threshold slider would be otherwise useless in this mode. So all you'd need would be an on/off switch, which could be just an extra value on the opacity control that already exists in the viewer window. (What do you call such a control? I'm having trouble coming up with the right noun.)
Getting the black outline around the supra-threshold voxels seems less trivial, though.