For those who haven't come across it, LetsEncrypt allows you to obtain free DV SSL Certificates but requires a server side script to be run periodically in order to renew the certificates (for better or worse, a 90 day expiration period has been used).
Although the provided script has plugins to allow support for automatically generating SSL certs based on NGinx and Apache configurations, the script assumes that the server is the origin and that the relevant docroot is available for writing to.
In the case of a reverse proxy - this won't be the case. We want the certificate on the Reverse Proxy (being the endpoint the client connects to) but the websites files are hosted on another server.
This documentation details a simple way to work around that on a NGinx reverse proxy (it should be possible to adjust the config for Apache's mod_proxy if needed).