I believe I understand what you're saying. Typical feather effects are generated based on bringing transparency into the center of the object, whereas you are searching for taking some of the values outward. BoltBait's old feather let's you do both of those. Though this can also be accomplished with a zoom blur to some extent.
I created a render of what I believe to be your desired effect by separating the truck object with the eraser, duplicating the layer with the clean edges so I had two copies of it, and using the zoom blur on the layer behind the foremost dupe. Because the zoom blur doesn't simply grow the object but smudges inwards, leaving the dupe in the front preserved the sharpness and integrity of the original object.