update on my experience
I mentioned I was going to try the standby upgrade from BKK-DPS. I did so and followed the hints given by some of you here in this thread. Happy to report all worked fine. It is a slightly slow process but after going to the Royal Silk counters at BKK I was asked to wait about half an hour (there had been an aircraft change) and then when it was certain they had a seat I went to the ticketing desk to pay by CC - the cost was 8,800baht... which is more or less US$250 as per the chart provided.
I enjoyed the flight and as I was feeling quite tired the comfort was very welcome.
On the way back I decided to stick with Y. It was a fairly light load so I had plenty of space. I'd like to have u/g again but with a lack of expendable funds I resisted the temptations.
I agree this service is really worthwhile doing if you can afford the extra or make a last minute decision to fly in a higher class. I'd certainly be doing it on long haul any time.