It would be great to see this resolved in Discourse itself.
Eliminating ad-hoc local filesystem writes in favor of persisting to well-defined backing services along the lines of The Twelve-Factor App would allowing easier deployments and horizontal scaling across different platforms. AWS Elastic Beanstalk may be a little less common, but the ability to run on Heroku seems a good litmus test for the deployability of an app.
Also, depending on this single local filesystem seems to be throwing out much of the benefit of the docker-by-default approach that I’ve been impressed Discourse is taking.
Given Discourse already depends on Redis, that seems like a decent candidate for storing runtime-rendered stylesheets. Or since S3 is already supported for uploads, that would be another option that doesn’t introduce new dependencies.