Double click on the ipod icon on the desktop. A setting window pops up. In your iPod "settings menu" just tick the box "manual synch". That means you always have to/can tell the iPod what to synch and what not.
This isn't an answer to your question, just a suggestion, but considering how much trouble you'll be going through to manage the music that way, you might consider buying another external hard drive of the same size as your iPod's drive and storing your complete iTunes library on it rather than on the laptop drive.
You should be able to find a decent 120 Gb USB external for around $75, less if you're patient and look for online specials.