Perhaps it would be sensible for Postfix itself to provide a configuration parameter that somehow lets you conceal or replace the sending IP/hostname of submitted mail?
My solution...
main.cf:
smtp_header_checks = pcre:/etc/postfix/header_checks_smtp
/etc/postfix/header_checks_smtp:
/^Received:/ IGNORE
/^X-Originating-Ip:/ IGNORE
