Yes, in some sense it's an obvious default, which seems to go largely unrecognized (even by those who understand thermodynamics), maybe due to a bias towards thinking that value scales linearly with material. But I don't want to claim too much. There are a number of caveats I didn't mention in my post:

• Some space-time geometries may have better entropy dumps than black holes. In an open universe without dark energy, for example, the cosmological background temperature goes to 0 and negentropy is essentially infinite.
• Why make negentropy the object of fair division, instead of value created from using up negentropy, which might not be a linear function of it?
• Why should individuals own matter? If they don't, then our intuition about what constitutes fair division would change drastically.

may take some explaining before it's equally obvious to everyone

That doesn't seem like a partly useful meaning of "obvious."

The real issue is that most people don't believe in the future. Do you want that as a prereq for game theory? does it have positive propaganda value? (I'd guess that it has negative propaganda value, but I'd also guess that the people complaining are incorrect about how distracting it is.)