You can already do this, just choose the option to inherit knot radius from particles, and set variation on your particles.
Because internally there is no notion of "subspline ID" in a spline object, it's not viable to add the variation within the mesher itself, since if subspline count changes that will affect the random seed of each subspline and cause jittering in the mesher if per-subspline variation is applied.
Because internally there is no notion of "subspline ID" in a spline object, it's not viable to add the variation within the mesher itself, since if subspline count changes that will affect the random seed of each subspline and cause jittering in the mesher if per-subspline variation is applied.