Usually works well to just show the status card while crediting to another FFP. However, maybe note so much for seat selection as you would then need to first add you status number to the booking - and whilst this CAN be removed/changed it is rather sticky and quite often messes up the crediting.
The only occasion I've had a serious problem was trying to claim my bag allowance at an outstation when crediting a different program. No matter in which order they did things (check bags in with FFP with status and then change program) the systems told them to collect money for my bag. In the end a supervisor told them to put a note of my BAEC Gold number in the override comment field - but took a good 20 minutes of discussions and problems.
So if you don't have a physical AA Plat card I suggest getting hold of one ASAP as it will otherwise be difficult to convince airlines of your status.