As I mentioned on f.ds, I was very much hoping to get this feature. As for what to do with version control issues with old logs, you might try asking the Prismata folks how they deal with it, especially considering that they rebalance the units quite frequently, something that doesn't happen in Dominion since it's based on physical cards. My best guess is that they keep track server-side of what all units did during each version, and include the version in the replay file. I don't know exactly what version issues there might be given that what the cards do never changes except to work correctly, but I don't know the underlying code base and how the log is put together.
