If the player won't be able to ever see it as a sphere (eg. by going to high altitude), you can fake it. Make 'normal' voxel world, but wrap it at the borders. Trying to load chunk [-1, 5]? Return [MAX_CHUNK, 5]. Want to visit [MAX_CHUNK+1, 5]? Oh, you mean [0, 5].
This way you won't get nice "hills rising from above the horizon as you get closer" effect, but it can be seen by the player as a normal draw distance limitation. You also will not be able to dig a tunnel through the core of the planet. On the other side, programming it is pretty straightforward and it should work as fast as normal voxel world.
You can add shader warping the models as if the world was a sphere, but observed curvature might not match world's size.