Sync a computer as an iPod: Is a way to sinc one computer's iTunes library from another in the way of an iPod available?
I would highly recommend looking into iTunes Match.

It allows you to sync all your music across multiple computers, iPads, iPhones, and iPods.  Not only that, but it will let you stream music that is in the cloud that you have synced and not on your computer.  This may be a good solution for you when you want to listen to songs but don't have them on your machine.  It is 25 dollars a year and it does require a internet collection to stream.

You can also just download a playlist of songs you want to listen to if you need to have the songs with you when you don't have an internet connection. 

It also syncs playlist together between all your machines.  Also, in iOS 6, it will allow you to stream the music as well. 

It also works with music that you have ripped and not purchased.  

You can read more here:

http://www.apple.com/itunes/itunes-match/

I know it isn't the most perfect solution but at least it will probably save you from a few headaches of trying to make sure things sync between the hard drive and your external.