Happy to report, that the Netlify team added a new development build to my website (basically the image for the building system used as far as I understand) and I can build with Hugo extended without adding preprocessed sites or the resources folder to my repo.
You can add yourself to the list at https://github.com/netlify/build-image/issues/182 - but don’t forget that it’s a dev version, that’s some levels more unstable than a beta. SCSS, pipes and minification are working (on my site). The beta they offered back in December was stopped soon after they started due to some issues.