Keeping in mind that everything you’re going to do will not be supported:
- Harder: You could try HAProxy. Depending on your configuration you might have to basically Man-in-the-Middle yourself.
- Easiest: You could try editing the nginx.conf file on your MIAB to proxy to your internal IP hosting your own ownCloud, for the /cloud/ directory on your MIAB. You would need to restore this modification to the nginx config after each MIAB update.
Also, you may well experience email delivery issues to other email systems if your IP PTR record does not match the FQDN you’ve set for your MIAB, as many email systems rely on forward-confirmed reverse DNS checks for verifying incoming email is originating where it claims to originate from. Moreover, many residential ISP net space blocks are blacklisted by Spamhaus as a matter of course.