Not from what I can see no. You can of course back it up to an external drive, but not change it's location. Not unless an Applescript would do it.
Technically, you can if you're willing to move your entire library to the external drive. I'm doing that with a NAS, and it works fine for music & apps. However, if you want to have your music on one drive and your apps on another, I agree that it's only possible (if at all) with an applescript, although I've never heard of this sort of thing.
