Just an additional note for the HKG lounges.

Yes, United Club just scans the boarding pass barcode (which is incredibly efficient!). Singapore Airlines SilverKris Lounge has their reception staff manually write FFP nos. into a logbook. There have been a few occasions where even though I was flying with them out of HKG and the check-in agent verified my *G card there (with the boarding pass having the "YOU ARE INVITED TO THE SILVERKRIS LOUNGE" and "CO*G" notations), when I approached the lounge without the card thinking that it wasn't needed anymore, I was still asked for it for the purpose of jotting down the details. At the THAI Royal Orchid Lounge, they have a scanner to take a copy of your card and boarding pass.

It's a very inefficient process with TG & SQ despite the technology readily available. Especially if you're travelling in a group and accessing the lounge, they'll have you stand there for a few minutes while they enter everything in (particularly the HKG SKL, unlike the BKK SKL where they grant you access first and then come back later with the cards and BPs).