I have been able to visit the LAS club on arrival, but it is always dependent on capacity. For the third party clubs, they are just interested in getting the revenue from PP by letting you in, so they are less likely to push back unless they are already full. Of course, this is really only useful for traveling domestically in the USA, as well as intra-Schengen, when you are able to access the departure area upon arrival and do not have any checked bags to collect. PP used to include access to a dedicated arrival lounge in HKG (Plaza Premium), but that lounge is closed due to the remodeling of T2.