Parece que, a partir de julho de 2017, IS é uma forma de alcançar isso agora: link
The ability to override the URL or IP where a request resolves to is now available in Page Rules. This is an Enterprise feature only.
A common use case for this functionality is when you are serving an application from the URI (e.g. mydomain.com/app). In this case the 'app' may live on another server and may even be hosted by a third party. Requests to this endpoint must be directed to the server for that third party application. You can specify a CNAME host.
Important: the CNAME record must exist within the Cloudflare DNS.