I don't follow. You still have to pay for erasing and changing your bits, regardless of whether you use reversible computing and do the erasure at the end, or whether you do it during the computation as in irreversible computing.
You generally uncompute intermediate results in reversible computation, rather than erasing them: if you produced some garbage by starting from a low entropy state and running the computation C forward, you can get rid of the garbage by just running C backwards (perhaps first copying whatever output you care about, so that it doesn't get destroyed).
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.