New status update: I've encountered a li'l problem. The filesystem on the server came pre-formatted with ext3, and it turns out that ext3 doesn't support more than 32000 directory links, which prevents me from copying all the accounts.
Currently, I'm looking for a way to detach one of the mirrors from the MD RAID array, reformat the detached mirror with XFS and then use it solely for game data. While I'd lose the redundancy, it would have the advantage of keeping the game data on a dedicated disk, so I don't really mind it horribly much.