Given that you explicitly don't want the safe to survive a reboot, is there any reason to use a persistent filesystem at all? The advantage of EncFS is that the files continue to exist after a reboot; a tmpfs would work for your session system.

The only disadvantage of a tmpfs is that if your swap isn't encrypted, your session data isn't encrypted if it's swapped out; this is already a risk if the programs you're using the session data with aren't carefully written to lock the data into memory. In any case, if you're worried about what could happen if I hit your machine with a LiveCD, you're already encrypting swap.