The LESS embed function in ResourceLoaderLESSFunctions does not support using the flipped version for rtl.
That means that if you use, for example, .background-image() (from mediawiki.mixins.less) it will only use the flipped version for the fallback (for browsers that don't support data URIs).
CSSJanus does not actually flip the image in this case. It just uses a filename ending in -rtl when the original filename ended in -ltr (see https://www.mediawiki.org/wiki/ResourceLoader/Features#Flipping).
This approach would require some refactoring of the CSSJanus class, so embed could call it on a URL or filename.