I have the answer

Here's how they could do maintenance while minimizing the playerbase's frustration with the downtime: Provide access to a limited functionality server during the maintenance window.

The limited functionality server upon activation would have read/write access to a subset of the user's data. Whatever is easiest to manage and integrate back when maintenance ends. Some possible examples:

1) Allow player pet battles.
2) Allow queing for random pve/pvp instances
3) Guild, trade chat.

It's not hard to keep track of the data generated by those features. What loot dropped and which lockout to update, etc. There would be no cross-player interactions allowed wrt money, items, AH, guild management, etc.

If the maintenance includes fixes to pet battles, disable that and still allow random queues, or vice versa.