This module processes scss files with curly brackets, but not .sass files with
indents.
Could it be possible to choose this in the admin? (Format: SCSS / SASS)

To enable it, you would only have to change from csspreprocessor.scss to
csspreprocessor.sass on line 20 in class.Preprocessor_SassCommand.php

One problem with this aproach, would be that it is not compitable with regular
css. And since all css are processed togheter, ALL css files would have to be in
.sass format. But it should be an option anyway.

Ofcource, the best solution would be to be able to select processing method for
each css-file. But I see this is a bigger project.