The reason Enhance doesn't support email piping is that it's a multi server control panel and most large deployments are likely to have separate standalone email servers. Aside from that, email and PHP are individually containerised (by design) and have no visibility of each other even if running on the same server.
We're unlikely to support email piping in the future. It's not impossible but it would need to be handled with an RPC which transmits the message "over the wire" to a daemon running within the website's application container which in turn calls the script to process it. That's a lot of engineering complexity.
If you need to process email messages programmatically (for example to facilitate a customer support/ticketing system) then it's usually more robust to poll the email server with POP or IMAP. If your script fails, the messages will remain in the inbox rather than filling up the email server queue or being discarded.