Hi,
I would say that no iRule would solve your issue and that's because its the DNS entry on the client machine that timeouts out after 30s (by default) which make the client send another DNS request to resolve the same name. The ECS is, for me, the only solution to your problem.
You can increase the TTL value for the DNS response and hope that the client make the next request within the timeout. However, the more you increase that value the less you benefit from inteligeante dns ...
Keep me in touch if you find a better solution,
Many thanks,
Karim