I am using the Cast Companion Library (because it makes implementing some of the design guidelines easier).
I do not want to use the VideoCastControllerActivity when playing video and instead want to use my own activity.
Does CCL facilitate this without modifying CCL?
You can define a TargetActivity that will be called from Notification, Cast dialog and Lock Screen.
If you just want to modify the look and feel of that activity, you can simply copy over its layout XML and change it as you see fit.
Finally, you can also implement VideoCastController yourself; that is what the VideoCastController does.