No the activity macro is not visible to anyone, so you can't modify it and you'd be forced to do something on DOM side.
What is the exact requirement with the confidential option? If there are people who are not allowed to see the content, then instead you could have them work on task level and copy the comments and values on the main ticket to the task and inside the script that handles the copying, you can make the script impersonate a "Anonymized" user to post the commets as anonymized.

Not the ideal solution, but it's something relatively easy to do and if there's confidential information on a record, it makes sense to prevent unrelated users from accessing such record completely.