OK, here it is.
I used HFVExplorer as you suggested. That HOWTO-Mac_Games is really quite useful.
Followed all the instructions and copied letting the program decide the format. Seemed to work fine.
So here's the result:
http://www.speedyshare.com/files/252358 ... demo_0.zip
Now, there's only something a bit weird there: when HFVExplorer copied the files, a .txt extension was added to all the resource files (behind their original extension). The zip file "discworld_mac_demo_0.zip" contains all files just as HFVExplorer copied them, with the added .txt extension.
But when you check the original disc on HFVExplorer, that weird .txt extension is not there, so I made another zip file taking out the added .txt extension from the resource files:
http://www.speedyshare.com/files/252358 ... demo_1.zip
So there you go. I don't know much about Mac files (first time fiddling with this kind of thing), so I've just given you everything I found. Hope that works.
If something's missing, or we need to re-extract the files in a different way, just tell me and I'll do it.
Oh, BTW, I've tried adding the game with the ScummVM SVN binary (putting all files in one directory), but it's told me it can't find any game there. I guess it's normal, if no one had access to this demo before!
I guess I should post a request to the bug tracker for the demo to be added to ScummVM, right?