I personally think this is impossible without manual intervention (although Im happy to be proved wrong)
Recently I d/l Itunes 8 for a new 16GB nano, and asked it to get cover art where it proceeded to apply the wrong art to some albums
The only way I could get to apply the right art after already having the wrong ones on the nano was to:
1) delete album off the nano
2) delete art from itunes
3) re-add manual found art in itunes
4) re-add track/album to nano
Recently I d/l Itunes 8 for a new 16GB nano, and asked it to get cover art where it proceeded to apply the wrong art to some albums
The only way I could get to apply the right art after already having the wrong ones on the nano was to:
1) delete album off the nano
2) delete art from itunes
3) re-add manual found art in itunes
4) re-add track/album to nano