Mathias Moehl from Mamoworld recently released his script CopyMask2Layer for After Effects, along with a tutorial on its use. CopyMask2Layer you are able to copy masks from one layer to another in After Effects while keeping the masks in place, even if the layers are moving and the mask paths are keyframed. Check out the CopyMask2Layer at Aescripts.
After Effects Tutorial: CopyMask2Layer (english) from Mathias Möhl on Vimeo.