you could try calling membership services ahead of time and asking that they watch your flights and manually key you in as Plat on the date that you'd make it, based on your iten. I did just this in a very similar situation a few years ago:
http://flyertalk.com/forum/showthread.php?t=452796
I talked to a very kind AAdvantage agent the week before, and explained that I would make Plat on the HKG-LAX segment. She looked at my iten, and she volunteered to upgrade me to Plat and rekey my iten while I was on the 14 hour LAX-HKG segment. So, the lesson is that you might not have to wait for your final flight to post to finish the Plat challenge, provided that you have a nice agent (like I did).
(OK, so I mistyped HKG-LAX--should have been LAX-HKG--but the point is the same.)
On the other hand, CX didn't realize the change immediately and I had a tough time in HKG when trying to enter the Wing/Pier. At least back then, manual changes did not push immediately to other OW partners.
two final points/questions:
1) do Partner EQPs still count towards AA challenges? I thought they changed this in 2007 so only AA flights count.
2) remember to have an agent re-enter your FF number on any existing itens so that your new status is recognized.