What you could do is to insert the protected content into a separate document to which you apply 'filling in forms' protection. Then, at the point where that content is to appear in your main document, insert a link to the protected document via Insert|Text from file>Insert as Link. That way, you can still use 'Track Changes' for the unprotected content. Although users can still edit in the linked area, it's impossible for them to preserve that content against the content reverting any time Word refreshes fields in the document (e.g. when a print preview is done or the document is printed). Provided the document containing the protected content is stored in a folder Word can access for all users, there shouldn't be any problems.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]