While it is really nice that you can home brew automation for all kinds of function with the SRX, this particular feature is pretty basic. Support for DynDNS on small branch and firewalls we deploy to the home office spaces of our employees is really a requirement for a business class solution.
We use these firewalls to connect the small office and home office users on a dynamic account with a VPN tunnel to company resources. There is really no need to consume static ISP accounts for a few people in an office or a home office setup for remote employees.
All the competitive equipment for small office VPN firewalls have DynDNS as a push button built in feature, including the Juniper ScreenOS line.
This really needs to be on the roadmap and put into some future release.