Sorry, I thought you wanted to check the status of a member in a pool. Are you wanting to just remove a host from the list of hostnames when it's out of service? If so, it would be better to define the hosts in separate datagroup and then update the datagroup when necessary. The advantage to this is that you don't need to reload the rule to initialize the change to the hosts list. You could still reference the datagroup using the list commands (or the findclass command). If you wanted to stay with the list in the RULE_INIT event, you should be able to just modify the list to add/remove hosts.
If I'm misunderstanding what you're trying to do, can you clarify more?
Thanks,
Aaron