All that mapping a drive in windows means is that when you want to access it you type the letter you have it mapped to, in this case Z: It's a shortcut by another name.
The problems are that sometimes windows drops shares for no particular reason, and i believe you take a small resource hit as there is an extra layer of network address translation, I might be wrong about the last, but that's certainly how it feels on a 2k3 server based network with a lot of servers, drives and shares. I usually get snappier response by entering a path direct to a share than using a mapped drive.
The NAS is effectively a standalone file server called Zeus. The path to your music files would be \\mshome\zeus\public\music. Z=\\mshome\zeus\public.
If this is all about wanting to just clone the library from windows, noting that the library is a bunch of references pointing to the music, then surely reassociating a 90gig library (I think that's right?) wouldn't be too bad, especially given the script file pointed at above to let you just do a straight re-import and then knock out the redundant links? You aren't going to have to feed the mac version all the music all over again, just let it look at what's already there and build a new library that will then maintain sync with the file store on the NAS independently and concurrently with the windows machine. Both copies of iTunes will behave like they have the music and noone else, and for that matter both should be able to play it at the same time, network willing. The NAS box itself is the shared resource, and the mac or for that matter any other machine that you care to add to the network really don't care that one pc that happens to be running windows thinks it's called Z:, they just see it as its share names and partitions and files.
My Mac based iTunes library, made with default settings, is arranged along the lines of \\iTunesMusic\artist\album\tracks. I would have thought that would make it a trivial task to just move the whole lot to another drive (in finder,explorer, via a stack of dvds or whatever) and then teach a new install of itunes or any other music playing software to find the music i wanted, couple of hours tops if I went through and did lots of hand edits.
Simply copy and pasting the xml for the library across the network isn't really the way it's meant to be used, but as this line from mine indicates, it's using standard paths to the files too... %20 is a 'space' (like ascii 32)
<key>Location</key><string>file://localhost/Users/marie/Music/iTunes/iTunes%20Music/Sonic%20Youth/Washing%20Machine/06%20Little%20Trouble%20Girl.m4a</string>