@wallabybob:
@BlueToast:
I found a problem with this.
The dnsomatic API documentation at http://www.dnsomatic.com/wiki/api says in the section Update Parameters "To update all services registered with DNS-O-Matic to the new IP address, hostname may be omitted or set to all.dnsomatic.com (useful if required by client)."
Ok, yes, this worked. Thanks!