Soft Mask with Type 1 (Function-based) Shading

I want to use a custom function as the soft mask - Type 1 (Function-based) Shading

But a function, does not generate alpha to be used with SMask, so my questions are

1. Is there a way to make a Type 1 (Function-based) Shading to return alpha?
2. Is there a way to transfer R, G or B layer into alfa?
3. If not, sould I use Soft mask with Luminosity?
4. Any examples somewhere of using functions as soft masks (with alpha or luminosity types)?

  Thank you!