I have been able to get whitespace-mode to display dots for leading spaces.
But I want to specially display each sequence of 2 leading spaces, with maybe a vertical bar.
Maybe not with whitespace-mode?
As a separate question, how do I get whitespace-mode to display only leading spaces as middle-dot?
whitespace-space-regexp "\\(^ +\\)"
...but that seems only to apply to the FACE used, not to the character substitution.
I've been fiddling with this for 20 minutes. I'm amazed that I still have not cracked this. Doesn't everyone want something like this?