They most definitely do (discretly) sell paid upgrades onboard. I enquired on check-in at BKK and when the crew arrived I was introduced to the purser who said she would arrange everything onboard.

On board she had a complete chart in her service manual which listed all the prices for Y-J and J-F by route. I paid US$800 for J-F BKK-SYD by credit card. Manual paper transaction, and no other invoice offered. When I followed up with the local sales office for an official receipt they could not help. I guess I could have followed up with Dubai but didn't bother. Only J Skyward miles were awarded.