the ECSS resource can be evaluated using createCompiledCSSResource method
but you need to call createResource method because of visibility issue.
However you need to obtain the instance of ResourceFactoryImpl first - it's not available from the context, so you can access it via ResourceHandlerImpl#createResource method which delegates to ResourceFactoryImpl instance.
RichFaces' resource handler can be obtained using FacesContext.getApplication().getResourceHandler();
At the end, you can use Resource#getInputStream() on created Resource object in order to evaluate ECSS resource.