OK, it seems like we’re at least getting somewhere now, Ajahn! 
Because you agree that nibbāna at least logically isn’t a self, that logically it is anattā. So if you don’t mind, I’ll break my promise of my previous reply being my last.
I introduced the unicorn just to clarify the logic, to make clear that the self doesn’t exist. But to many people—in a sense for everybody but arahants—it is not so clear that the self doesn’t exist. So the comparison with a unicorn only goes so far, and likewise with the BMW. When it comes to the self, people actually have “lost the plot”! Hence “it just sounds weird” is true for the unicorn and BMW, but for the self I disagree.
The question then becomes, is it possible for at least some people to wrongly see a self/I/mine in nibbāna? It seems the suttas indicate that it is. I mentioned MN1, which is very direct. Also there is DN15, where the absence of all vedānas—which is just the same as the absence of all viññāṇa or all phenomena—is taken as a self. So apparently yes, people can take this as a self. Hence there may also be a practical need to call it anattā.
Perhaps such a view makes no sense to you, but I would argue that in that case it is you who may be relying too much on logic. Because when it comes to a self/my/I, the mind just isn’t logical and will look for it everywhere, even if it defies logic.
But regardless, who is to say that with sabbe dhammā anattā the Buddha wasn’t just making sure he was being logically consistent? Sure, the dhamma is about more than logic, but it also doesn’t contradict logic. In fact, it is very logical.
To say sabbe dhammā anattā would also have been useful for other reasons, perhaps especially for people that did not understand nibbāna. Because if the Buddha would just have said sabbe sankhārā anattā then by analogy of the previous two characteristics, people could easily conclude that the asankhata was a self. In fact, that seems a very logical and natural conclusion to me. And I realize now is probably the main reason I think sabbe dhammā should include nibbāna.
We have to keep in mind here also that most people listening to this would not have understood what nibbāna is. With that in mind, we can also take the teachings on anattā in a different way. If people believed that nibbāna was a kind of eternal mind, that would effectively be a self. The Buddha to indicate that it is anatta, would have countered such ideas. What I’m getting at is, to say that nibbāna is anatta is effectively almost another way to say it is exactly the absence of all phenomena. Because any kind of existing phenomenon would be worth calling a self.
