Hello,
The "Hair From Guides" doesn't work with maps when "Guide as Hair" Mode is used.
That will be a necessary feature, because we often need to reduce hairs to drive some fonctions.
For example, we can want to reduce a baked hairs comb to generate guides, and use these guides in a "Clump by external guide" applied to an istance of this bake hair comb.
We also need to reduce a hair comb to few guides to animate the hairs with "Adopt external guides" operator.
And when we use these fonctions, It's better if it's just a reducing of the original hairs number than a re-interpretation produced by "even" mode for example.
So, If "Guide as Hair" could be simply reduced by a grey texture for example, it will be greatfull !