Sync from one Mac to another, treating second Mac like an iPad?

It's funny how the Macbook Air has been marketed as taking the best from the iPad, but it's still missing the most fundamentally successful feature of the entire iPod/iPad line: library management.

My MBA is a secondary Mac, like I think most of them are. It does not have 3.5TB of attached storage, and doesn't have room for my entire photo, music, OR video library.

To cope with my iPhones 32GB limit, I just plug it into my "big" Mac and the "best" 30GB of content is automatically pushed in using the brilliance of smart playlists, and all of my metadata carries over; also I can buy more content on my iPhone and have it sync back into the library.

I want to do that with my Macbook Air. Or frankly with any secondary computer.

Obviously this should really be a feature in iTunes- "slave mode" or I'm sure Apple would call it something strangely elegant yet bland, like "Manage iTunes Library from another computer."

But since there is no Apple feature, is anyone aware of either a piece of third-party software, or a somewhat manageable manual way to do it?

Again, my goal is to 1) Have about 15-20 GB of my best playlists sync from my Macbook to my Macbook Air, along with metadata like play counts and star ratings, and
2) Be able to easily sync changes to metadata and newly downloaded songs back to the mothership.

I could imagine being able to handle the files themselves with rsync or something, but the real trouble is the metadata in iTunes.

I am looking for a similar solution for iPhoto or Aperture, but I doubt that it exists. I'll settle for itunes.