I would be very cool if there was more options than just the sphere influence area than just the ellipse there currently is, maybe a rectangular shape, like in the image but that goes to infinity on both sides (or not) or mayve a triangle, i dont know, but the rectangle would certainly be a benefit,
You can do this quite easily. Simply select a LINE of vertices (or a rectangular area of vertices), then roll your mouse wheel to reduce the size of the influence circle. As you move the selected line of vertices, you will see that the affected area is rectangular. Try it and see if that works for you.
Also, remember that if you are deforming a mesh using the Proportional Edit Falloff tool, you can first select and hide (HKey) any vertices that you want to remain unaffected by the deformation. Once you are finished, use ALT+H to show the vertices again. They will be right where you left them, unmodified by the deform.