I just created this plugin that is a very basic UI Slider (similar to the jQuery slider), but it doesn't just allow you to move the handle horizontally and vertically, it will follow any shaped curve! If you've ever used Adobe Illustrator or CorelDraw then you probably recognize the Bezier curve below.
The handle, or grip as I call it, can be dragged along the black curve. For now, it only returns values from 0 (green dot) to 100 (red dot).
The plugin is in the early stages of development and still needs a lot of work, but it is usable now.
Check out the demo page, and the builder page. If you would like to help me out, submit an enhancement or issue, or fork a copy of the plugin on Github and send me a pull request! I'd love the input!