I usually have a folder somewhere that contains all the elements for my project that aren’t video. Like, photoshop files, music, etc.
Like you, I think iTunes is an excellent way to store music library songs. Even though you can’t drag right into FCP, I think this works well:
Once I find the file I want, I drag it into that project folder I have. Then, in FCP, I import that copy from my project folder. This is much easier than finding the file where iTunes keeps it becuse my project folder is much cloer to the “surface” of my computer.
I know that on paper it doesn’t sound much different than going to find the file in the hard drive, but in reality it only takes me a few seconds to do so I think it’s much faster. Plus, if I back-up that project folder when I’m done with the project I get the music along with everything else.