I don't think that grip strength is the main point of this movement. There would be a tremendous amount of force on the plates from pulling in on them just to perform the movement, which would negate a lot of the downward pull of the weight, thus taking the thumb out of the equation.
Here is a video of Shelby Starnes doing them, notice how little weight he is using to achieve the desired result of the exercise. I can't see his grip giving out on this any time soon.http://asp.elitefts.net/qa/default.asp?qid=120739&tid=
I have seen him go heavier with an EZ curl bar and he makes a point to use plates with a grip 'hole'. That also leads me to believe that grip isn't the target of this movement.
It seems to me that pinch-grip strength would be better trained by pinching a couple of 25s or 45s together and pulling them straight up off the floor.
Give the Reeves DLs a shot and maybe I am completely wrong. Let us know what you find!