First thing to understand is that SD (standard definition) PAL is always 720×576 which is just by the pixel count 4×3 aspect ratio.
How to we get the 16×9 aspect ratio images out of that without changing the pixel count. Simple, just define each pixel as NOT square and I can then create a 16×9 picture with a 4×3 pixel count = PAL SD anamorphic. The pixel count for PAL SD with SQUARE pixels is 1024×576.
Animation codec (or any codec for that matter) doesn’t care if your pixels are square, anamorphic, round, triangle… A pixel for the codec is just a bunch of numbers. The only reason some codecs have the 16×9 option is to tell the software that uses it that it’s pixels are supposed to be interpreted as anamorphic shaped.
So you can export from Motion as animation 720×576 and when you open this clip in FCP make sure to tell FCP that this clip is 16×9 and it will display it accordingly.
Hope all this makes some sense (if anybody disputes anything please correct me as I’m not a technician by trade)
Best
Carsten