Just my cents:
- a TTL of 1 is not conform to RFC's (at SOA's base, maybe allowed for an A record), you insert a task to froxlors cron so it takes just therefore more than 1 second
- your solution misses IPv6 support
- DNS is a fundamental service an should be high available, your updateip.php isn't
But all in all a nice thought an easy, lightweight solution implemented in froxlor. Thanks for sharing this.