The createDialog command does not work properly during the briefing. I use a diary entry similar to this one ...
player createDiaryRecord ["Diary", ["Create Dialog", "Click <executeClose expression='createDialog ""MyDialog"";'>here</executeClose> to create the dialog."]];
... and it works properly during the mission, but not during the briefing. The dialog appears to be opened in the background behind the briefing display (leading to visual glitches), but it is not shown.
Using findDisplay 52 createDisplay "MyDialog" instead of createDialog works properly during the briefing.