SASS for module CSS?
By concept a contrib module will hardly ever be able to use SASS, because SASS simply is not part of Drupals distribution. But that doesn't mean your local environment cannot use shared SASS resources for custom themes and custom modules.

E.g. in all our projects, the project-specific theme is a sub-theme of a shared, immutable base theme providing defaults and mixins. We are compiling with a bash script using standalone SASS with the --load-path argument:

sass ./web/themes/custom/PROJECT/scss/THEME.scss ./web/themes/custom/PROJECT/css/THEME.css --load-path ./web/themes/custom/SHARED_BASETHEME/scss


in ./web/themes/custom/PROJECT/scss/THEME.scss

@import "default_settings"; /* lives in SHARED_BASETHEME, bunch of $variables with !default */
@import "settings"; /* lives in PROJECT, partially overrides default_settings */


I'm quite certain there is something similar for the Node-Version of SASS (or in Gulp). With a black belt in in Bash-Fu (or in your case: Gulp-Fu) this can be extented to any custom module and any custom theme and vice versa. 

Example with multiple themes:
We had one project using Domain access module and one sub-theme per sub-domain and we handled bulk theme compiling with a bash script:

for D in ./web/themes/custom/*/;
do
  if [ -d "${D}" ] &amp;&amp; [ "${D}" != "./web/themes/custom/SHARED_BASETHEME/" ] &amp;&amp; [ "${D}" != "./web/themes/custom/EXCLUDE_ME_2/" ]; then
    echo "Creating sub-theme ${D}"
    sass ${D}scss/THEME.scss ${D}css/THEME.css --style expanded --load-path ./web/themes/custom/SHARED_BASETHEME/scss
  fi
done