Steve, I think you’re sort of out of luck.
If it’s a curve, you’re even more out of luck.
The only real option I’m aware of is to go to each keyframe and make the adjustment on a keyframe by keyframe basis. If it’s a circlular window, you can easily copy and paste. other than that, it’s quite a bit more tedious.
No promises on this, but maybe try duplicating the node in question (by copy and pasting it into another node) feeding that as a matte into the current node, inverting it, and then adjusting the settings in the key tab. It’s worth a shot.