You shouldn't need to repitch for carbonation, assuming you're brewing it like your normal beer. In fact, its probably easier with fewer cells, less of a pellet in the end. As far as I remember, you need to keep rotating the bottle, maybe once a day or so, until it pellets in the neck nicely. I'm sure there's an optimal angle for the pelleting to occur as well. This is just what I'm pulling from my memory of a winery that made champagne this way, they had a guys who's sole job was to go down to their cellar and rotate the bottles. I suppose it wouldn't be too tough once you had a setup ready to go.
Have you given any though to how you plan to freeze the pellet in the neck? Dry ice/ethanol might be cold enough to get it to work. I'd also be curious if there's a certain level of carbonation you have to reach before you've got enough pressure to push the frozen pellet out. Champagne gets pretty bubbly, not sure if a 2vol carbonated beer would have enough force behind it.