Very nice! So you don't just cheaply compute-and-uncompute a lot of independent worlds, you can allow them to leave an arbitrarily-difficult-to-produce trace on the future worlds. Given how much entropy we really have, sufficiently small persons for example can be spared from uncomputation.
In particular, a person can live in an incrementally computed-and-uncomputed virtual world that is being regularly reversed to its initial state, with the effect that only the person consumes entropy, and the whole arbitrarily complicated world has zero entropic footprint. The world could also be optimized game-save-style over person-time, starting from an initial state, but going forward, so that some version of the person does all the updating, and so carries the excess entropy. Alternatively, improvements to the world could be carried out by discarded copies. Think of software downloaded from the distant future...
Or more generally, this is just time travel, where you can transport sufficiently small things (and people) to the past (or between timelines) and change things the next time over. You travel forwards in time by computing, backwards by uncomputing, you can take some luggage with you, you can climb up a different timeline by observing without interference, and you can intervene and change a timeline any time you want. The network of people navigating baseline network of virtual worlds builds up a second level (implementing meta-time), which itself can be navigated and intervened-in by other observers, and so forth. Sounds like a Greg Egan novel (that wasn't written yet).
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.