Ghost - a couple of things you mentioned got me thinking about my setup and I think I now have it figurred out
Please note the text below reflects the setup on MY iMac
OK - after some searching and playing, here's what seems to work for my iMac...
1. Make a backup of your music library - just in case :-)
2. IN iTunes PREFERENCES : Advanced tab
2.1 set the Library to the directory(folder) of the NAS Drive - in my case it's set to "/Volumes/Volume_1/iTunes Media"
2.2 Ensure the "Keep iTunes Media Folder Organized" box is checked
2.3 press OK/Save
3. In SYSTEM PREFERENCES: Users & Groups
3.1 select your User profile and click "Login Items"
3.2 add the folder containing music - in my case I added "Volume_1" (just the NAS Drive Volume)
3.3 Make sure the little lock(bottom left) is unlocked when you do this and lock it afterwards
3.4 initially, make sure the "Hide" checkbox is unchecked
(NOTE:- it displays a Finder window on startup once the connections to the NAS is established - you can change this back to HIDE later)
3.5 - once that window has started you can start iTunes without any prompting for a library
3.5 Once those changes are complete, close iTunes
4. RESTART iTunes: while holding the "Option" (or Alt) key, start iTunes
4.1 - a dialogue box will request the location of the "the Library" - here's where it gets a little tricky...
4.2 DO NOT select (in my case) "Volume_1/iTunes Media" as selected in 2.1 above
4.3 Simply select (in my case) "Volume_1"
4.4 For some reason, this dialogue recognizes the lower levels of folder/directory.
4.5 stop/quit iTunes
5. STOP iTunes FROM STARTING AUTOMATICALLY - Ensure that iTunes is setup so that it will not start up during the logon process.
5.1 this is because during logon, the folder added in 3.2 above is allocated to the system and it takes time to link to the NAS
5.2 once the folders have been "assigned" a Finder window will be displayed
5.3 once the window(s) have been displayed iTunes can be started.
5.4 If you don't wait for the Finder dialogues to be displayed, iTunes will ask you to select a Library again
Once setup like this iTunes will point to the NAS drive without reverting back to the "default library" even after powering down
- a power failure will start iTunes before the NAS is allocated, so just quit and then restart after the Finder window for the NAS is displayed
That's is - At least on my machine :-)
As I said above - BACKUP YOUR LIBRARY FIRST!!!
Please note that some of this narrative may vary depending on which NAS drive YOU have