There is an example when the prime ministers of Great Britain and the dominions got together during the imperial conferences, where they came to a number of agreements about how Britain would interact with the dominions. They signed some formal agreements about how those processes would play out. One of them was that dominion ministers would be the ones to advise the King on who would be nominated and appointed as governors general in the dominion. The first precedent came when the Irish did that, but it was quite clear that everyone knew beforehand that's how it should be done.
So I have argued in previous work that you may not necessarily already need a precedent for there to be an obligation that exists. Those imperial conferences in the 1920s created a set of conventions where it was clearly understood that obligations occurred before the first formal uses of them.