P:(Masking) Make custom mask group re-usable for substract or intersect

Sometimes the (great) "select subject" tool does not get the subject quite right, and I will need to refine the subject selection with other masking tools (brush, color range, etc). 

I will give a name to this refined subject mask, such as "my custom subject".

I wish I could subsequently refer to that mask, when doing substractions or intersections with other masking tools (as if that mask was a variable).

Here is an example scenario that I am going through ffrequently:

1/ I use select subject

2/ I refine the selection, and name it "my custom subject"

3/ I do the treatment I want on "my subject" (usually I increase the texture a bit).

4/ I want to correct hot-spots created by my flash on objects brighter then the subject: I will create a radial mask, with the intention to reduce highlights, but I don't want highlights reduced on my subject.

Feature request: I wish I could substract "my custom subject" from the radial mask, the same way as I can click "substract" and then pickup "subject", or "sky".

Why this would be valuable to me: currently, in my above scenario I will spend a lot of time re-finding the subscracted subject, since "select subject" didn't properly identify subject edges in the first place.