Is there a "rotate colors" to alpha

[Ofnuts]

Once you have mapped the range to a (narrow) color, you can use C2A on that color?

Consider also that:

• the  Color and Fuzzy select can do selection on criteria other than plain color: hue, saturation, specific color channel
  
• you can make a copy of your layer an optimize it for a selection (contrast, etc...) and then use that selection to operate on the initial layer.