But maybe unrelated to this there is a mechanic called Demesne which allows the Emperor to give one of his conquests to the Empire instead of keeping it for himself. That province will change hands when the Emperor changes. One of the advantage is that the province has its revolt-risk basically lowered to zero so in some ways it behaves like a core but never becomes one.
PS yay, 100 posts!
PPS my latest update is at the bottom of the previous page ;-)