This can indeed be done.
In a Button field, it is possible to read out the mouse position when the user clicks. This can then be calculated as a percentage of the field width or height, and then calculated into the ranking.
It is not quite a slider, because it requires an action to trigger the reading out of the mouse coordinates, but it comes close to. I have implemented this concept in a color selector, similar to the ones used for selecting colors in Acrobat.
Hope this can help.
Max Wyss.