To construct a friendly AI, you need to be able to make vague concepts crystal clear, cutting reality at the joints when those joints are obscure and fractal - and them implement a system that implements that cut.
I don't think that this is true. Reductionist solutions to philosophical problems typically pick some new concepts which can be crisply defined, and then rephrase the problem in terms of those, throwing out the old fuzzy concepts in the process. What they don't do is to take the fuzzy concepts and try to rework them.
For example, nowhere in the "Free Will Sequence" does Eliezer give a new clear definition of "free will" by which one may decide whether something has free will or not. Instead he just explains all the things that you might want to explain with "free will" using concepts like "algorithm".
For another example, pretty much all questions of epistemic rationality are settled by Bayesianism. Note that Bayesianism doesn't contain anywhere a definition of "knowledge". So we've successfully dodged the "problem of knowledge".
So the answer to the title question is to ask what you want to achieve by banning porn, and then ban precisely the things such that banning them helps you achieve that aim. Less tautologically, my point is that the correct way of banning porn isn't to make a super precise definition of "porn" and then implement that definition.
To construct a friendly AI, you need to be able to make vague concepts crystal clear, cutting reality at the joints when those joints are obscure and fractal - and them implement a system that implements that cut.
There are lots of suggestions on how to do this, and a lot of work in the area. But having been over the same turf again and again, it's possible we've got a bit stuck in a rut. So to generate new suggestions, I'm proposing that we look at a vaguely analogous but distinctly different question: how would you ban porn?
Suppose you're put in change of some government and/or legal system, and you need to ban pornography, and see that the ban is implemented. Pornography is the problem, not eroticism. So a lonely lower-class guy wanking off to "Fuck Slaves of the Caribbean XIV" in a Pussycat Theatre is completely off. But a middle-class couple experiencing a delicious frisson when they see a nude version of "Pirates of Penzance" at the Met is perfectly fine - commendable, even.
The distinction between the two case is certainly not easy to spell out, and many are reduced to saying the equivalent of "I know it when I see it" when defining pornography. In terms of AI, this is equivalent with "value loading": refining the AI's values through interactions with human decision makers, who answer questions about edge cases and examples and serve as "learned judges" for the AI's concepts. But suppose that approach was not available to you - what methods would you implement to distinguish between pornography and eroticism, and ban one but not the other? Sufficiently clear that a scriptwriter would know exactly what they need to cut or add to a movie in order to move it from one category to the other? What if the nude "Pirates of of Penzance" was at a Pussycat Theatre and "Fuck Slaves of the Caribbean XIV" was at the Met?
To get maximal creativity, it's best to ignore the ultimate aim of the exercise (to find inspirations for methods that could be adapted to AI) and just focus on the problem itself. Is it even possible to get a reasonable solution to this question - a question much simpler than designing a FAI?