Feathering any shape

I am looking for a robust way to feather the edges of any polygon.

I have read every forum post available on feathering, and so I am familiar with the multitude of possible ways to feather basic shapes (circles, ovals, rectangles). However, it would be tremendously useful to be able to feather non-convex polygons, or, even better, to feather any possible shape.

An example of this would be the ability in After Effects to draw a mask of any shape, with sides that can be either lines or arcs, and feather it all the way around.

Has anyone worked up a way of doing this in Jitter, or as a shader (whether it’s frame-rate-friendly or not)?