No, I think that would be a very bad idea. First, applications
should not have to worry about their decorations. Second, when
the application finds out the expected size of the decorations,
the information may already be obsolete. And third, the window
manager may not be able to find out the actual values before the
window becomes mapped. This may depend on many properties of the
window which the WM can not find out beforehand, for example the
MWM hints, whether the window is transient for a valid, mapped
third window, its name, class, resource or icon name, whether the
window is mapped as an icon (and the WM allows icons at all), the
window group it belongs to, the position of the pointer, the
screen it is mapped on, and so on.
All these problems can be avoided by a "place frame at" protocol.