Especially with the new 3D view and texturing enhancements this would make a lot of sense.
A brush, driven by Python and/or OSL. When it is applied to a curve it follows it. When it is brushed on, it can leverage additional parameters like tilt/pressure and speed/velocity.
Similar to the procedural texture layer it should have a few examples and then let the user pop out a text field to paste in (and modify) their own code and attach variables to UI controls.
This could be made available for fill-layers as well.