This exists as an internal tool for use in testing review queues on the dev tier. It probably doesn't exist in production, and if it does it wouldn't be very useful: the current design of the review system would result in posts being removed from the queue as soon as the next synchronization task (runs every 5 minutes) realized they weren't eligible to be there.
So I'm declining this for the simple reason that it can't be implemented in a useful manner without some substantial re-working of the current review architecture, and "could be useful" isn't a sufficiently strong argument for such an undertaking.
But that's boring...
If this could exist, would it be useful then?
Gilles notes that you can accomplish this for certain review queues with the use of a sockpuppet. This isn't entirely true; not all reviews can be triggered by a flag, and all review queues have various qualifiers that prevent their use in situations where reviews would be impossible or unproductive...
But for the sake of discussion, let's ignore that and pretend there's a special "enqueue review" flag that will put a post in any queue the flagger chooses, overriding all other checks. You, the sockpuppet-wielding moderator, can now throw up any post for review whenever you feel like it. When would you use it?
Here are the scenarios that've been thrown out in this discussion so far:
When a borderline on/off topic post comes along and it's better to let the community decide whether it should be in scope or not.
This was ArtOfCode's rationale in the proposal itself.
To avoid using sockpuppets and looking shady.
Distilled from Gilles' answer and the resulting comment discussion.
Allow moderators to find potential help fixing it.
This was Wrzlprmft's rationale in his answer.
And here are the reasons they're wrong:
If it's better to let the community decide whether something is in scope, then let the community decide - don't put your fingers on the scale and pretend justice is still blind. If you feel a given post should be discussed then bring it up on meta; otherwise, leave it be and see if anyone else cares enough to flag or vote on it.
If you're doing something shady, then having an official tool to do it with doesn't make it any less shady. And I say this as someone with access to both lots of tools that are ripe for abuse and a dozen sockpuppets created for doing stupid, dodgy stuff. Sometimes, you gotta step over that line a little bit... And when you do, it's best if you remember that you're in dangerous territory and don't get too comfortable with it. If you find yourself having to sneak around trying to influence the community's decision without putting your name on it, then perhaps it's best the system doesn't give you a fig leaf.
It would be really nice if there was a way of queuing things up so that folks could pitch in without stepping on each other's toes. Complicated retagging efforts, answers that haven't aged well, broken links... I'd personally love a tool like this. Of course, none of the existing review queues are suitable for such tasks, so a general purpose 'add to queue' tool wouldn't do anything here; what Wrzlprmft really wants is an entirely new type of review queue, one that can be turned on for a specific set of posts with specific goals and guidance.
So, even if this was technically feasible, it'd still be a bad idea and I'd still be declining it. But, there are the seeds of a more useful tool to be found in that W person's answer.