YEC on its face is false, and YEC steelmanned into something that could be true, for some tortured definition of "true", simply doesn't pay rent. Ham has basically admitted that his belief in YEC is not entangled with reality. Any attempt to steelman his position into a philosophical position by which YEC can be "true" leads directly into solipsism. To believe that there is such a thing as reality, and reality can be perceived by humans, is to reject YEC.
Creationists and evolutionists, Christians and non-Christians all have the same evidence—the same facts.
It's hard to come up with an interpretation of that that is true. Clearly, Ken Ham does not have the same facts in his head as I do. Perhaps in some sense he has the same facts available to him as I do. Putting his claim in LW terms, he seems to be saying that he simply has a prior of P(Bible is true) = 1. But it's simply a flat-out lie that YEC is just a matter of different priors. For instance, look at this page: http://www.christiananswers.net/q-eden/edn-thermodynamics.html . This is not merely a matter of "I agree with science about the facts", it's "I'm going to write a bunch of bullshit to make a claim that quite simply is not true". As just one example, Ham presents the case of a dead plant not being able to use sunlight. He purports to prove a universal quantifier (in no case can energy being added to a system increase complexity) by presenting a existential quantifier (there is a case where energy being added doesn't increase complexity). This is both invalid logic (you can't prove a universal quantifier by proving a particular instance) and based on a false premise (even a dead plant, when exposed to sunlight, increases in entropy [ETA: I didn't phrase that quite right. A dead plat, when exposed to sunlight, increases in "complexity", at least for some meanings of "complexity". Part of Ham's strategy is to use words with contradictory definitions]). Ken Ham doesn't disagree with scientists about what 2LOT says because he has different priors, he disagrees because he's a liar.
Furthermore, look at how this debate came to be in the first place. There was a video on YouTube of Nye saying that YEC shouldn't be taught to children. AiG replied with a video saying that YEC should be taught to children. You can't "teach" priors. Either someone has the same prior as you, or they don't. If you give an argument for your position, you aren't changing their priors, you are changing their posteriors. The very fact that Ham believes that YEC can be transmitted from one person to another shows that he has implicitly admitted that this is not a matter of priors, or "presuppositions", or whatever he wants to call it. You can't debate priors, Ham agreed to a debate, ergo Ham's YEC is not a matter of priors.
In a recent discussion of steelmanning, I observed that few people around here seem interested in steelmanning young earth creationism. In the resulting subthread, someone suggested Yvain's steelman of the Time Cube as illustrating what a steelman of of young earth creationism might look like. But steelmanning the Time Cube may be too much of a stretch, it's a little too incoherent to steelman effectively without changing it into something else entirely. In contrast, once I thought about it it wasn't hard to come up with some ways to steelman young earth creationism in a way that was very much in keeping with the spirit of real young earth creationist writings.
The way to do it, I think, is to approach it from a philosophy of science angle. For example, here's a quote from an article titled "Creation: 'Where's the Proof?'" on Answers in Genesis, a website run by young earth creationist Ken Ham (who recently debated science guy Bill Nye):
You find a lot of this kind of thing on Answers in Genesis. For example, they're willing to concede that on certain assumptions, radiometric dating is a strong argument that the earth is a lot more than 10,000 years old, but they deny that they need to accept those assumptions.
I honestly don't think it's much of a stretch to steelman this into something that would look a lot like some of the things the philosophers of science I studied in graduate school said. I started writing a long post spelling this out, but then I started worrying I was going too far in playing devil's advocacy for creationism (even for an exercise in exploring the weaknesses of steelmanning). So instead, I'll just mention some places too look for material in such a project: The Duhem-Quine thesis, confirmation holism, underdetermination of scientific theory. Fun fact: Pierre Duhem regarded the existence of atoms as a metaphysical question that could not be settled by experiment, and this has not stopped him from being regarded as an important contributor to philosophy of science. I suppose Feyerabend belongs on the list too, but that's almost too easy (even though, yes, I did have to study Feyerabend in grad school).
Oh, and I could even find material for my steelmanning of young earth creationism in the writings of Robert Pennock, a philosopher of science who testified against Intelligent Design at the Dover trial. Some philosophers, while thinking creationism is dead wrong, have criticized the reasoning used in that and other court decisions that have kept anti-evolutionism out of public schools in the US. Pennock wrote a response, titled "Can’t philosophers tell the difference between science and religion?" where he argued, among other things, that methodological naturalism (MN) is essential to science and supernatural claims are inherently untestable. A relevant quote:
So on Pennock's view, testing young-earth creationism and thereby demonstrating it to be false is not possible without relying on naturalistic assumptions. This creates an opening for the creationist to question whether science needs to rely on naturalistic assumptions, and argue that one could create an equally valid version of science based on (fundamentalist) Christian assumptions.
You might conclude Pennock is wrong about this, and young-earth creationism really has been straightforwardly refuted by science, but this creates a different opening for the creationist: argue that if Pennock is wrong, his ideas really shouldn't be the basis of court decisions about whether creationism can be taught in public schools. Evolutionists could respond by arguing for some other philosophical basis for rejecting creationism, but then they'd probably have to make some contentious philosophical claims and we shouldn't be determining what children can learn based on contentious philosophical claims either.
An argument along the above lines could also be used for a different purpose, by someone who rejected creationism but wanted to make a show of being fair-minded towards their opponents and generally more rational than most of their peers. The thing to do is to say that while young earth creationism can be decisively refuted, most scientists and philosophers botch the philosophy required to do that, and this indicates their rejection of creationism is mostly tribalistic, and the young-earth creationists don't actually come out looking so bad by comparison.
Speaking as Chris Hallquist and not some hypothetical alter-ego, I think that if a philosophy of science makes young earth creationism come out looking good, that's a reductio for that philosophy. I think educated people who reject young earth creationism are generally rational to do so, even if their philosophy isn't that hot. Still, I'd be curious to know what else people on LessWrong can come up with in the way of steelmanning young earth creationism.