I just tried this, hopefully its what your after, I'm not sure what the performance would be like.
You can mask an effect by doing this:
1. Create a white mask in its own sprite. (White will be where the effect occurs, Alpha 0/Transparent what you don't effected.)
2. Give it a Multiply effect. (this effectively hides the white mask, make sure this is the first effect).
3. Give it the effect you want. In this case blur. (I did 2 blur horizontals at 300 and 100 to address the blur quality I was getting.)
4. Place this sprite on its own layer (or with other 'post-processing' effects) and anchor it in place.
Multiple effects can be added, but I found not all effects play nice with this approach.
The mask doesn't have to be a large image to cover the screen - it can be a small image scaled up... in your case a 1 pixel wide gradiated white-transparent-white band could be scaled across the layer, again I'm not sure what gains you the best performance or if its even relevant.
You could conceivably animate the mask sprite for additional undreamt up things you could do with it... or call specific frames of a mask for various needs.