There are several aftermarket VVT cams that are tuneable, PWR, Arrington, and Inertia just to name a few. the only problem is they are all mild cams, becuase of piston to valve clearance. A few have tried bigger cams and had tuning issues, but I am not sure why. Some companies are secretive about their VVT cam specs, so comparison betewwn them is impossible due to lack of specs.
As for the 6.4 compatibility issues, only time will tell when someone actually attempts to install these items on a 5.7.