Making local masks or local corrections is not currently possible in RAW processing. But it occured to me that it would be relatively easy to assign a graduated mask over the entire image to apply a certain conversion setting for one side, and a different setting for the other side. if the gradation and rotation are adjustable, it would solve a lot of the local masking problems that photographers are faced with. What do others think?
If, in addition, the source image could be different for each side, and/or the grad mask is a blurred luminance mask of some sort (like shadow/highlight correction), then it could even become possible to do bracketed composition without resorting to PS...
You should post this at Adobe's forums as a feature request - there must be a lot of us who want it. A poor substitute, and more handy for portraits, is to drag the vignetting slider to the left, increasing the vignetting and burning in the corners. The maths behind an ND filter isn't too hard, and while ND might not make v1.0, make your case and it might not be too long before it's there.