Ok I've had another experience this week, not as glowing as the last one (and more consistent with what sparoz has experienced) - but I still got the result I wanted.
I was looking for a J redemption seat on a particular flight SYD-HKG just over 2 weeks out. It became available on the Asia Miles website.
I called up the call centre and got a guy who would not hold the seats for me without the points being in the account. "Can not".
So, I transferred points over (interestingly the same seat was available through QF for 65,000 points, or through CX direct for 45,000 asia miles... typical).
The points took 4 days to move over from Amex, rather than overnight. I've been told they are sent Mondays and Thursdays US time, meaning they appear in our CX accounts Wednesdays or Saturdays.
By the time the points were in my account, the redemption seat had annoyingly gone, so I decided I'd call up and give it a go.
A lady answered and told me there was no availability that day, but she could book me the day before. I asked if she could release the seat and she said "can not".
To be fair, the loadings for the flight are pretty full, Economy in all fare buckets for the entire day (17 April) is completely sold out on all four CX flights.
So (Krisflyer style) I just decided to call again and get a different agent. This one also told me she couldn't release the seat, so I said "I am Diamond and you've done it for me before". Instantly, she said "Ok let me see what I can do, please hold". 3 minutes later I had the seat confirmed.
It seems as though it's a bit luck of the draw as to if you get a good agent or not, and you may have to push them to do it... but even when they say they can't, they can!