Caddy allows the responses in reverse_proxy to be intercepted via handle_response like so:

php_fastcgi unix//run/php/fpm.sock {
    # FPM needs full application root
    root /application/public

    @accel header X-Accel-Redirect *
    handle_response @accel {
        root * /files
        rewrite * {rp.header.X-Accel-Redirect}
        file_server
    }
}

This allows us to send a "private" file from backend that is available locally without having to stream it through PHP.

I haven't found it in the docs (but maybe didn't look carefully enough), can this be done in FrankenPHP?