Why not just bury new comments at the bottom of the thread until they get an upvote (essentially equivalent to "endorsement".) It's a terrible idea (new comments never get seen), but it's functionally the same and only a small modification rather than a whole new system.

You can also have an option to "hide" comments below a certain threshold (i.e. ones that haven't been "endorsed" yet) if that is the desired system. Or an option like showdead for low karma comments (perhaps count only total upvotes or "endorsements" rather than upvotes-downvotes.)