I just don't buy into the idea that bowling in 50 overs games will make Craig a better bowler in the longer form. It's true that his biggest weakness is his ability to be a holding bowler when conditions aren't in his favour, but performing a holding role with a Test field against Test batsmen is so, so different to bowling in a limited overs game.
The best path to improvement when it comes to performing a holding role in Test cricket is to have him bowl in situations when it's required more.. and lots of First Class cricket in New Zealand seems pretty much ideal for that. The blokes who actually do manage to perform a holding role in the first innings of Tests don't bowl their limited overs lines and trajectories when they do that; it's a unique skill within itself.
More cricket the better. There will be times in test matches where he has to bowl, and the likes of Warner on a 100 and Clarke on a 50 are taking him easily for 4-5 runs an over. Where you can forget about short leg and silly point and a slip surrounding the bat on day 2. Where that scenario is far more similar with a spread field to bowling overs 11-35 of a List A game, where he must contain, and where the pitch is not responsive to turn.
Sure, he can flight the odd delivery up, but against test star red hot batsman looking to take him on, learning to contain List A aggressors will be incredibly useful for him. The class of batsmen will be less, but their intent will be the same, and the pitch may be just as unresponsive. Sure, in the test mid on and mid off are hopefully not on the boundary, but bowling to Warner, a captain may just put them long. He is not selected to get wickets in the first innings, that is Boult and Southee primarily (and whoever their support act is - Wagner, Henry, Bracewell). He does need to crop rotate overs though because Boult and Southee cannot bowl unchanged.
When the pitch is unresponsive on days 1-3 (outside Asian say Asian conditions), I wish he could bowl 2 runs per over darts like Nathan McCullum did at FC level if he so wanted or was directed to by the captain, and then go for runs and wickets on days 4 and 5. I'd prefer that he had a variety of deliveries like Vettori, sure, that prohibit the batsmen scoring cheap runs quickly on days 1 and 3 and still get the odd wicket. Who wouldn't? He can also learn to implement those in List A cricket when batsmen are trying to score off him. It would be a very useful learning curve. it can only help to develop his game. I don't believe that the 50 over game is so dissimilar in all aspects between overs 10-35 to the red ball game for it not to be of service to him in test cricket. That is the key difference is our thoughts. I would change next to nothing about his day 4 and day 5 approach because he is figuring that out for himself just fine and dandy and delivering quite nicely.