I wonder if anybody knows if there's any such filter that would do the opposite of a fisheye lens filter and only work horizontally?

A filter that would take an image like this (A round shape to illustrate better):



And kind of stretch the outer horizontal pixels and result in an image like this:



Pegleg

Maybe Liquid Rescale?

Wouldn't that just be supplying a suitable negative value to the first parameter of "Filters->Distorts->Lens Distortion"?