In the case of a top-level comment or a reply to the journal owner (not to another user), the ability for the commenter to post a screened comment could be useful, and the journal owner could unscreen it if they wished. The commenter shouldn't be able to change the screening later, as that leaves it open to abuse. Also, this way, commenters would be very aware that the journal owner can unscreen their comment, so there would be no false illusions about security.

I'm not so sure about how to make it work in communities or when replying to someone else, as you would then need the "screened" comment to be visible to more people (the person you're replying to as well as you and the journal owner, or the mods as well as the entry poster).