So an update on this.
I’ve successfully fooled the system. I’ve forced a Content-Length header by overriding the StylesheetsController which prevents the chunked response, working around Ember CLI’s bad habit and avoiding nginx’s off-spec allergy.
How I got to this solution was by noticing another Ember CLI/nginx setup over https, where Rails was choosing not to chunk the Stylesheet responses (anyone know why this might have been the case?). That lead me to try to force the header and, waddayaknow, it worked!
This is only needed in development, so I’m not too worried.