Curving an Horizontal Slider

Yes but you'll have to code it yourself using GUI API's and objects

You can use a Sin wave if your curve is a semicircle. Just adjust the value inside de Mathf.Sin, the example provided is in javascript, and is useful specifically for what I needed. But you can have a start with it and modifie it to your need.

 hSliderValue = GUI.HorizontalSlider (Rect (940, 135+(Mathf.Sin(hSliderValue*-.0175)*30), 243, 30), hSliderValue, 0.0, 180.0);