I use two Applescripts to achieve this effect.
First, I create a playlist on my iPod of all of the tracks whose info I want to sync back to my iTunes library.
Then using an Applescript called
Mirror iPod Playlists in iTunes, I copy this playlist over to iTunes. It is important that you ensure that the resulting playlist in iTunes has the same amount of tracks, as the next step won't work unless both playlists are exactly the same, including play order.
(Handy Tip: if you want to make the play order of a playlist to be that of a column you have selected, just right-click on the playlist in the Source list, and choose "Copy to Play Order".)
Then I select all of the tracks on the iPod version of the playlist, and use
Copy Tag Info Tracks to Tracks v1.4. This will copy the metadata that you select from a popup window from the tracks you chose, and then lets you apply it to whatever other tracks you wish. In this case, it's the duplicated playlist in iTunes.
It's a bit more tedious than simply having iTunes set to automatic, and it requires you to have all of the songs on your computer already, but it gets the job done. Make sure you read all of the documentation for those scripts carefully.