It doesn't say in the mook if you can hold down the button. I'm making an educated guess (basing on the fact that A Tager Driver and B Tager Driver both have notes regarding being able to hold down the button) that you are not able to hold down the button to extend the duration of 720C. That aspect of 720C was incredibly stupid anyways in CS1, and if it's confirmed that it's gone, I'm glad it is. That move is fucking retarded.
Oh, and I believe 1080D also follows suit.