You should be able to do it on board if there are seats available.

I've done it on the DXB-(BKK)-HKG route mentioned in the post above. You can only do the current sector this way, but as it's your final sector that's not an issue.

It was for me as I wanted to do right through from DXB-HKG but could only do it as far as BKK upon boarding at DXB.