sym linking iTunes Media -> Music to external thumb drive
You can actually change the location that iTunes stores its library under Preferences -> Advanced -> iTunes Media folder Location. 

In that case, you could migrate your iTunes library to that external drive. You would just need to make sure you have the drive connected when wanting to use iTunes.

You may want to be careful with carrying this much on a thumb drive (if they even make them that large). As in, its easy to loose, get damaged, etc - so make sure you have a backup. 

Another good way to do this would be to just use an external hard drive connected over firewire or USB.