I am not using sass extensively, but I know it is possible to inject global variables directly in the loader. Here is a quote from the entry page:
If you want to prepend Sass code before the actual entry file, you can simply set the data-option. In this case, the sass-loader will not override the data-option but just append the entry's content. This is especially useful when some of your Sass variables depend on the environment. You may find more information here: https://github.com/jtangelder/sass-loader#environment-variables.
As I said, I am using less most of the time, so this is the workflow I am used to.
You are right, webpack compiles sass first. You can see the config in cfg/defaults.js:
You may want to try out a different configuration there. Hope this helps.
Please feel free to open this issue again if the problem cannot be solved.