Needs a bit of research and a bit of work, so will take some time.
I might just restructure it so that,
Allow Guests to Post => Allow "whatever" group to post.
Allow Guests to Comment => Allow "whatever" group to comment.
The "whatever" group can be anything you would find in the permission list you have when, for example, you assign access permissions to a module.
I have also added this to
Bugger Feature Requests.