It's just like if you had a full board and one of them had a curse in the middle stone. If that comes up "draw ghost" you'd still have to draw it unless all spaces are full. And you place it by it's color as normal.
So, in other words, you always lose Qi if you're full up in the Yang phase; but being required to add a ghost in the Ying phase only loses you Qi in unusual circumstances (such as the whole board is full or the curse die says so). Am I getting it?