Effectively, in order to make custom application tool bar, you can only use the functions "TOOLBAR_Set" or "TOOLBAR_ChangeButton".
And "if application want to to use the full screen", there is no solution using CircleOS and CircleOS API, and you have to write your own functions.
Ntrf, I don't know what about you thought about "solution", but this behavior is not forseen be changed with new versions of CircleOS.