I like this goal.

" * Private topic posts have a "private" indication where "important" and "whisper" would normally go. This may conflict with "important" which is used by the warnings addon."

I like the idea of taking off whisper from private topics. But don't see a need to change "important" or add anything else.

You could change the label on the post reply button(s) to something like "private reply". And use different icon(s) on the button.

Automated response: Make private topics visually distinct from public topics

This hotfix highlights private topic posts in a slightly seeded amber.

A hotfix (a TAR of files to upload) has been uploaded to this issue. Only apply this hotfix if you absolutely need it and cannot wait until the next release of Composr (releases are more reliable and strictly tested). As of Composr version 11, the recommended way to apply a hotfix is by following the same steps as an upgrade (https://baseurl/upgrader.php, use the hotfix on the step “Transfer across new/updated files”). The upgrader will automatically skip files belonging to addons you do not have installed or that are newer on disk than in the hotfix. Otherwise, you can manually extract and replace these files (do not replace if your on-disk file is newer than the one in the hotfix). Always take backups of your site or at least files you are replacing before applying a hotfix. Not sure how to extract TAR files to your Windows computer? Try 7-zip (http://www.7-zip.org/).