Tom,
I have to tell you that those changes are non-trivial. I've wanted to add the dimension feature myself, and haven't because of the amount of work involved. But then I'd have to do something generic (inches, mm, cm...), and come up with a way to do it in the existing UI with minor changes. It certainly can be done, but it won't be easy.
I've got the CS2 and CS4 versions here. Not the CS3...
I think the CS3 version included the metadata options in the ContactSheet script itself (if so, it would be around line 2900 (I don't have the exact line number).
If it's not there, it's in WasScriptLibrary3.jsx
The captions are compiled in Bridge, and then sent to ID via BridgeTalk. You'll probably want to do your magic where the captions are put together. See ContactSheet.parseCaption(). That's the function I call up at the top around line 95 (give or take) that generates the caption for the individual image based on the caption template the user creates in the UI.
Hope that helps.
Bob