are you sure, Michael?
Most of the built-in feather and matte effects can only do the same feather all the way around.
well, i'd say ALL of them actually, but it's a long time since i needed something like this.
it'd be good to be wrong on this.
i think there might be an Alex4D or Andy Mees plug-in that can do one side.
another approach is to create a gradient in Photoshop or similar and use it either in an image mask filter,
or as a compositing layer,
your best and most immediate bet could be to look at what MOTION has to offer.