DynDNS
Using DynDNS is not a viable solution for your usecase since its primary use case is mapping a dynamic ip address to a dns record. If you have two Upstreams with different ip subnets you would need to edit ip address or nat confiuration on failure which could be scripted but is prone to error.
GSLB
Another way of using DNS would be GSLB, which utilizes DNS to load balance and use a keep alive method to check for availability. Since DNS is bound to TTL you will always encounter a certain downtime since clients will cache the ip address to fqdn mapping and it will need to time out so it might be the wrong solution for you.
BGP
Using BGP to solve the problem would be the optimal solution. You would have an AS and your own ip address range which you advertise to both providers. If you encounter issues with one provider the peering between your local router and your isp would time out and traffic will automagically be routed to your other provider (I will not get to technical, there is still more to it)
Considering Complexity... BGP is not complex to set-up in this scenario. You would still need an experienced network engineer to ensure best-practice configuration but your scenario does not require much BGP knowledge.
Considering Cost... Annual costs for AFRINIC & additional costs of ISP peering with you. You would need router(s) to be BGP capable but nowadays there are even small HP/Cisco/YouNameIt routers that are capable of BGP...heck even Sophos UTM is BGP capable. :)
Hope I could help... I tried to be generic and not go too much into the details, drop me a note if you need to know more.
Best Answer
I'm quite sure there is no such command or feature in TiMOS. The one way you can emulate such behavior is with a static blackhole-route, a prefix-list and a policy-statement. It's not what you asked but still pretty decent. Sample configuration:
Now you can apply that policy as an export policy in BGP group level before your normal export policy.
edit: Of course advertising default to your CE device without knowing if you have upstream connectivity could be considered as poor design. One way to enhance the configuration above is to make prefix-list of known dynamically learned prefixes from your network (or internet) and make those static blackhole routes conditional. This can be achieved with
prefix-list <name> [all|none]
parameter in the static-route context. By default, logical OR is used.