Ah, the $64,000 question.

Not saying that you need to do all of this, and there are several options, so if I accidentally say "you need to" I'm not trying to say that this is the onl way.

You need to (doh!) draw up a strict rota for adding music to the iTunes libraries - this applies if you use OS9 and OSX and you reference an iTunes library on another Mac through a shared drive. So it may be - add to OSX first, then OS9, then other Macs.

I don't normally let iTunes organise the music for me. I keep it all on a (slow) non-boot disk, and organise it as I like, so all soundtracks go into an "Original Soundtracks" folder, incomplete albums into a "Various" folder, compilations into a "Compilations" folder, audio books into "Audio Books", etc, etc. Of course, the soundtracks are all in folders by film or stage show name. Keeps the length of long lists of artists down. I could probably have done a better job by subdividing the rest into "Classical & Opera", "Rock", "Oldies" and "Other". It just keeps the folder sizes down, ie no folders with 600 more folders in it. My point is that I CAN do this because I don't let iTunes organise the music for me.

And because of my strict rota, and preventing iTunes organising, I can have a folder hierarchy like:
iTunes
= 00 iTunes Music
= 10 Load on MacPro
= 20 Load on OS9
= 30 Load on OSX


So new stuff goes into folder 30..., I load it to iTunes in OSX and move the folders of music into the 20... folder. Next time I'm in OS9 I'll add them to iTunes there and move them to the 10... folder. I can move iTunes music and videos, as long as it stays on the same physical disk partition - the iTunes library contains the full filename with path for each track, and the inode, and when I "move" files on the same disk, the inode is unchanged. When I finally add them to the Mac Pro iTunes library, they're moved to the 00... folder where all the rest of the music (140GB) is already located.
I can move the folders from the Mac Pro immediately after I import them into iTunes.

Having the lame numbered folders just makes it really easy to find and import and then move all the music folder that needs it, ie. everything in the folder.

Caveats:
well, the iTunes artwork is sometimes stored in theiTunes library, rather than embedded in the mp3, so importing like this misses that.
and if you're in a hurry to get stuff into an iTunes library without following the strict rota, it can be a pain, but then it's just that your rota is not what suits you.

There are alternatives, after importing into one iTunes library, import the actual iTunes library file into the other iTunes libraries. This means removing all the existing music before importing and this will also hose any playlists you have meticulously built, unless you're clever with smart playlists.
That started out as just using the same library file - http://support.apple.com/kb/TA44499?viewlocale=en_US - but as iTunes for OSX went up in versions, and the metadata changed, you needed to import the library file. Of course, you'll lose any metadata not supported in v2.0.4 of iTunes.

Maybe someone else has an ingenoius method of doing this.

just get an iPod?

Or get a cheap £30 dedicated Mac (G3 / G4), put an OSX version on it and run it headless. It will run iTunes for you with all your music. It wil also have the only speakers connected to it, unless you want audio for your games through extenal speakers also.

As said, run it headless with an old Apple Remote Desktop, v2 is pennies on eBay, or use Chicken of the VNC. you can connect to it, fire up iTunes and start music playing. I do something similar already for all the tasks I don't want "burdening" my PMG5 (as if anything could burden it).

Not really a solution, but a work-around. It's just a bit of an effort, but can become irritating if you don't switch into OS9 or OSX all that often.

Just hit me just now, don't know if it will work, but you could keep all the music in iTunes v2.0.4 and always run it in Classic. Of course, you'll miss all the new functionality, but music is just music.