Let me try to parse and respond:
Yes, it seems possible that a superintelligence embedded in the world can exert enough control to reverse the universe's computation. I don't think density is the relevant notion, and there are confusing issues going on here (which have certainly been touched on at LW). It is not clear that this sort of reversal is possible.
Yes, it seems like reversing the universe's computation may be useful under a broad range of circumstances, though ai cooperation is not high on the list for me--recovering negentropy the universe has already burned seems much more likely, and incomprehensible superintelligence-activities seem more likely still. I agree that recovering as much negentropy as possible may require cooperation between not-identical AIs.
I agree that if you have some sort of causal/decision-theoretic-significance account of consciousness, the possibility of these interactions might give you a Born rule. But it seems like an account of consciousness has already done most of the work of explaining the Born rule, and this is sort of an afterthought. I think it is possible in principle that consciousness in our universe only exists because of some convergence of this form, but again most of the mystery is coming from the account of consciousness (and similar strangeness is possible without QM).
I don't have a good model for why you write the things you write (though I rarely vote them down). LessWrong seems like the only place anywhere on earth where people care about some of the things you think about it, but your behavior suggests you don't care about it. I accept that this is due to facts about you I don't understand, and that comments like this one may just make the issue worse, but it does seem to me like the stuff you write about isn't at very much inferential distance from the zeitgeist here, and that if you wanted to talk about it uncryptically you just could, and if you don't want to talk about it it is perplexing that you bother.
Density is important because entanglements run away at light speed: if you can't get those entanglements back then you can't reverse the past, you can only reverse back to the point where the superintelligence originated, which isn't that neat. The only way to recohere the system is if there's a boundary condition or, more understandably, if a superintelligence catches your lost information and rushes it back to you (and presumably you would do the same for it, so it's a very clear-cut trade scenario). Problem is that it might take a very high density of s...
This post may be interesting to some LWers.
In summary: it looks like our universe can support reversible computers which don't create entropy. Reversible computers can simulate irreversible computers, with pretty mild time and space blowup. So if moral value comes from computation, negentropy probably won't be such an important resource for distant future folks, and if the universe lasts a long time we may be able to simulate astronomically long-lived civilizations (easily 10^(10^25) clock cycles, using current estimates and neglecting other obstructions).
Has this been discussed before, and/or is there some reason that it doesn't work or isn't relevant? I suspect that this consideration won't matter in the long run, but it is at least interesting and seems to significantly deflate (long-run) concerns about entropy.