First, only one dynamic DNS client is wirking at the time. Maybe, something changed in last F/W, but it was before.
If you want to update some dyndns account (not exactly DynDNS.org), you can use HTTP poster. It can be configured to post something every some minutes, but there is a different problem. For example, DynDNS.org will block you for too frequent update. As fact, HTTP poster can't be used for this service.