Turns out the model is NOT supported. 'Generation' numbering differs, so what is sometimes called "third gen" is other times called "4th gen". And the 2GB brushed metal versions are so new they're not even mentioned on the gtkpod website.
I had to resort to running iTunes in Windows XP in VirtualBox. Even there, the latest iTunes crashes, I had to get version 9.2 (or was it 9.0.2?) from oldapps.com. I also needed the commercial version of VirtualBox - the OSE doesn't support USB devices - and that in turn needs libpng1.2; my distro had 1.4. (I wish I'd known about oldapps to start with, I might have been able to run an iTunes version in Wine, rather than using a VM which runs like a dog on my ancient system).
Nightmare, absolute nightmare. I now hate Apple with a passion. Fortunately it wasn't my iPod, so I won't have to do it again.