I've always longed for the ability to put custom items on the toolbar—to attach MCC commands and whatnot. I suppose a Toggle([Field]) function, or a more universal Increment([Field]) (which would toggle a boolean field), would be a nice embellishment.
But I wonder, why are you not using [Last Played] to automatically determine movies not yet watched? I realize that runs the risk of changing the status of a movie not fully viewed, but surely that's not a common occurrence—or at least not one that causes a problem. I imagine the movie you didn't finish watching last night easily found at the top of your "Watched" view. But if you're concerned about it, you can use a rule that compares [Bookmark] to [Duration] to decide which view such movies belong in. Or just clarify the issue by displaying something like
If(IsEqual([Bookmark],),,FormatDuration(Math([Bookmark]/1000)) // [Duration]).