We do already have the url_for function with a cachebust option which can append a ?h<sha256> path to the url. Does that solve the hashing for things coming from static? It also does work to get a link for a given content, not just assets.
The point of having the processed assets in the static folder is that they are committed in the repo and do not need to be reprocessed unless they change, putting them in a tmp folder would be annoying for me.
I’m not sure I like putting more asset processing in templates though. Images are a natural fit there because that’s where you are using it but sass?