By having a customized trust level for “see and review” new messages, forum operators could set that to any level - level 1 by default and mod level if they want to lock it down.
Though I’m no fan of symmetry for symmetry’s sake, I’m not seeing why this kind of approval inherently requires more trust than flagging-to-hide. In short - if a group of users can be trusted to provisionally hide stuff - why can’t they show new stuff as well? Anything “inappropriately” approved using this process can still be hidden by flags or moderator action.