I have probably stayed around 70 nights at the JW Marriott Bangkok, 21 nights in Nov/Dec 2004, and most definitely, I have changed the temperature on every room from 9th floor to exec floors. You need to press the button for about 5 seconds (up or down) until the temperature flashes instead of the clock.
I have been able to pick up the Landmark Club signal for free on the landmark side and SMC 2 signal most times on the ploenchit center side. Worth a try if you want to avoid the charges, but obviously depends on location.