Specify default visibility for comments

We have projects that are visible to external customers and internal staff. Most of the comments on an issue should only be visible to internal staff, not to customers, but the default visibility for a comment is "All Users". It's error prone to need to remember to change this to the group for all internal staff - it would be better to be able to specify a default group for comment visibility, either by project (with fallback to "All Users" if a given user is not in this group), or by user.