Looks like a neat solution.
But you CAN already very effortlessly auto-synch a bigger library than your iPod, using iTunes alone. You just use iTunes prefs to say WHICH playlists to synch. And it can be done so that NO songs are excluded forever. My method, for, say a 1000-song iPod mini:
1. Synch a smart playlist of your favorites--by rating or by most-played. Choose a number (400?) that fills the iPod partway with songs that will always be there.
2. Fill the rest of the iPod with a smart playlist of the least-recently played tracks (perhaps excluding the lowest-rated). This gives you the entire "rest" of your library. Yes--all of it! The least-recent technique means that every time you plug in your iPod, songs you've heard will automatically be dumped and replaced with other songs. (But your "favorites" in the other list never get dumped.") So while your iPod randomly picks tunes from its own HD, the iPod HD in turn is always picking new songs from your larger library.
It's like having infinite songs on a finite iPod.
To improve this method even more:
3. Include a playlist of recently-added songs--I'd always want those, even if they aren't "favorites" yet, because it takes me time to decide how well I like new songs.
4. Divide the least-recently-played list into 2 or more, so you can emphasize certain songs (by rating or genre maybe). Such as: 50 1-2-star songs, plus 200 3-4-star non-classical songs, plus 50 3-4-star classical songs. Even if your collection has more 1-2-star songs than 3-4, and half classical, you could have your iPod emphasize 3-4 stars/non-classical--without totally excluding the rest from rotation.
5. Include some (maybe all) of your non-smart custom mix playlists. You made them for a reason, so you should have them! And they will have a lot of overlap with your "favorites" so it won't take much space to store them.
For example:
* 400 top-rated songs
* 100 recently-added songs
* 50 least-recently-played songs, 1-2 stars.
* 400 least-recently-played songs, 3+ stars
* 50 most-played songs (you may not think of them as your favorites but stats don't lie: these are ones you tend not to skip)
* And a dozen custom mixes
Those would overlap a bit and just about fill a 1000-song iPod mini--with an ever-changing selection from your larger library that will automatically bring EVERY song into rotation eventually--plus your favorites and mixes always there. Your iPod auto-synchs from now on with zero effort, and your huge collection "fits" on a mini!
And you can have as many other playlists as you want too--they just can't all synch. That's determined in iTunes prefs.