Irule that waits?

I don't think it's possible to know what the load-balancing decision is/will be in the CLIENT_DATA event. What you might be able to do instead is leverage the LB_SELECTED event, where you can use the LB::server command. I don't know exactly what you're now looking to do, but one possibility is to record the SessionID in CLIENT_DATA, but do all your selection logic in LB_SELECTED, overriding the load-balancing decision if your array already has an entry in for the SessionID.