"Seems like a simple idea would simply be to change just the spelling of one seemingly insignificant name near the end of each version of the credits and then by actually watching the credits you'd know the leaker straight away."
"Of course, you'd leave the watermark on there so the leaker doesn't know how you caught them and thinks that the watermark is still the tracking mechanism."
I liken it to the Simpsons or Futurama in the sense that things subtly change (e.g. locations which are destroyed in the show, are destroyed in the opening, and such).
I think you can tell I'm a fan!
You want to make it so that the entire binary content of the film looks different for each person, so there can be no point of reference apart from what it looks like (which is the same).
But yes, once it was discovered that information was being embedded in the credits, those would be cut.
But in this wide world there are fans who painstakingly go through the credit and put the text online. Sometimes even special thanks. So a fake name could be detected, and once the m.o. is detected, in this networked age, everyone would know it and people will start looking for it.