In the limit where you have exponentially more time than space (say, the universe turns out to be some arbitrary reversible bounded cellular automaton) then entropy does no good at all.
Ok, I see, but this assumes that once you've completed a computation, a second execution of it has no moral value, right? (Because more negentropy would allow you to drive the reversible computation forward faster and complete more executions in the same available time.)
Yes--if over the course of your computation you explore on a fraction X of all possible states of the computer, a supply of infinite negentropy would allow you to run the computation something like 1/X faster.
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.