We boarded the Bliss on Dec 2 from San Pedro. We didn't see any Latitude tiers on any signs. The CAS and The Haven lines were the first lines, then I saw a Priority Access line but it didn't elaborate if that included the usual Latitude tiers or it was the paid PA. After that it sorted by deck numbers, half in one and half in the other. Our deck line was short so I justed jumped in that and we were checked in within 10 minutes.
I assume that you'd use the Priority Access line for the Platinum embarkation perk. If our deck line was long I'd have tried that.