Sure. Put the border on the top layer (make it 600x429) and in the publish settings just export the movie in 640x480.
Note that 600:429 is not the same ratio as 640x480, so it might get stretched.
That is exactly what I cannot allow to happen. It must
not be stretched even at the correct aspect ratio, because I am not using vectors but raster images; all of my backgrounds are at 600x429 resolution. Stretching them would completely ruin the look.
I don't really know how Flash works that well, but might it be possible to create a new .fla file of 640x480 resolution, and import my 600x429 .swf file into it?
For that matter, I have another question:
Is there any way to make sure that a flash symbol is placed EXACTLY in the centre of the frame?