Semi success!
Totally ignoring the installer, I unpacked
The Sims3.app folder, dragged it to the Applications folder, and edited the following files :
\Applications\The Sims3.app\Contents\Info.plist <key>CedegaGameDir</key>
<string>D:\The Sims 3/Game/Bin</string>
<key>CedegaGameName</key>
<string>D:\The Sims 3/Game/Bin/TS3.exe</string>
\Applications\The Sims3.app\Contents\Resources\Preferences\config[Drive D]
"Path" = "\Volumes\Games"
"Type" = "hd"
"Label" = "Games"
"Filesystem" = "win95"
By double-clicking the app, the Launcher appears. When I click Launch Game, it asks for a CD.
I went to the terminal, navigated to
\Applications\The Sims3.app\Contents\MacOS and ran
.\ciderA window appeared with the green plumbob. Then the screen went blank, and I got roughly 5 seconds of the loading game music. Then nothing. I had to reset the computer because, being a n00b, I didn't/still don't know the keyboard combination to restart.
Now, the drive I have it installed on is actually NTFS, and currently read only. I wasn't expecting TS3 to demand write privs, so perhaps that's the problem.
Anyway, I'll go back to OSX a bit later once I finish chatting to some birds.