I'm ashamed to say that I didn't figure this out myself. It is, after all, consistent with the behavior across the rest of the system.
I have iTunes configured such that it does not copy songs into my library as they are added. This allows me to split the library across multiple hard drives while also conveniently having a much more frequent backup schedule for the metadata.
When adding new songs to the library via drag-n-drop, I often want the newly added songs to be consolidated into the master music repository.
This turns out to be really easy. Simply opt-drag the songs into iTunes and it will copy the songs into the library instead of just adding 'em "in place".
Duh. Just works.
Andy Boyko asked how I have my iTunes library setup and how the above interacts with the setup.
I have two primary listening locations; home and office.
At home, I have an external 160GB drive that is the music library (i.e. iTunes is pointed to that location in its preferences). There are two partitions on it; 140GB primary and 20GB secondary. The primary partition contains all the music. The secondary partition is used for BitTorrents. Specifically, I BT a lot of live music from ETree and use the secondary partition to preprocess the tracks.
For encoding of CDs (I still have about 2/3rds of my collection to encode), the encoded songs are automatically inserted in the library. Fine. Which is good that it is fine, because I don't think there is another way.
For addition of already encoded songs -- DJ mixes and live music -- I will add them directly from the BT volume or the desktop. They stay there until I have decided they are worthy. At that point, I simply hit consolidate library and delete the originals.
The office environment is a little bit different. I have a 48GB 2.5" drive that I don't trust for historical reasons. I wrote a little AppleScript that copies 40GB of random tunes from my home listening library onto the removable drive. It then becomes the primary source of music in the office environment. iTunes keeps all the library metadata on the machine's local drive.
When new tracks are added to that machine outside of what is on the external drive, I always keep the new tracks separate so that I can easily keep track of them until they are integrated into the home library.
This includes purchased music.
I wish all of this were easier and that there was a way to share metadata between the two environments. But, frankly, what I have now works well enough for the time being.