This I did by mounting the NAS's drive and creating a Music folder on it. I then repeated some of my previous steps—launched iTunes with the Option key held down, created a new library within my ~/Music folder on my startup drive, and pointed iTunes Advanced preference to the Music folder on the NAS's drive. This time I enabled the Copy Files To iTunes Media Folder When Adding To Library option as I wanted to be sure that when I added files to my iTunes library, those files would be placed on the NAS. I then chose File -> Add To Library, selected my now-organized and culled media files on the external FireWire drive, and went to bed rather than wait several hours while the files were copied over. The next morning, there they were in iTunes, ready to play.
I could have now left things exactly as they were. One of the features of this particular NAS is that it offers an iTunes Server. When this is switched on, the NAS will appear under the Shared entry in iTunes' Source list. Simply select it and the music the NAS holds appears, just as if the songs were on your local hard drive. However, if you want to create playlists, you must do so within the NAS's interface inside your Web browser. I prefer to manage this kind of thing just as I would if my media files were stored on my Mac. Plus, using this scheme, you can create different playlists on each Mac. So, dad can have his playlist full of Jimi Hendrix hits, mom can rock out to Zep, and child can ignore mom and dad's old tunes and tune into Adele.
I then went to each Mac and performed these same steps save one. Now that the files were on the NAS, all I had to do was mount the NAS on that particular Mac, choose File -> Add To Library, and navigate to the Music folder on the NAS. No files were copied to the Mac. Rather iTunes simply added the tracks and video file names to the iTunes library. Over a wired ethernet connection this can take an hour or more for a large library. Over a wireless network, it takes significantly longer.
The final piece of this particular puzzle was to ensure that the NAS mounted each time I restarted my Mac. This I did by launching System Preferences, selecting the Users & Groups preference (choose Accounts if running a version of OS X prior to Lion), unlocking my Administrator's account, and clicking on the Login Items tab. I then mounted the Music folder on the NAS and dragged it to my list of login items. On restart the NAS mounted (I was prompted for its password before it did so, which occurs with each restart).