Interesting - when I was hacking up the Raw.plist file to make 10.4.4 support the 350D I found that adding data would cause the OS X image to turn black (in particular I was trying to see if adding an entry that the Digital Rebel XT had that the 350D didn't helped the colour cast I got). This lead me to think that Apple are using position dependant decoding of the data structure rather than parsing the file 'properly', but I have no hard evidence for this. All of the files I altered with ExifTool viewed without a problem in Canon's DPP software and when decoded by dcraw, so the modifications are not themselves incorrect.

If you can change existing data but not add new items, then I think it's time for a bug submission to Apple as they have obviously only just started messing with RAW and have a lot of catching up to do (not that I'm still sulking about the lack of non-US camera support. That would be petty).