Can't think of any other way than create your animation, then duplicate everything, and in frame 0 switch off fills in one version and lines in the other, then render each version.
In version 8 there is a new feature in the 'Project Settings' called 'Render Style'. Set 'Fill Style' to 'Background' (and leave the rest at 'Normal') to render only lines in a tiff or png sequence. To render fills only set 'Stroke Style' to 'None' (and leave the rest at 'Normal').