Now that the 48-hour downtime has begun for "selected realms", which unfortunately includes my home turf of Magtheridon, I feel I can release this delicious orange of wisdom for future downtimes.
Blizzard GMs have confirmed that auctions do indeed continue during downtime. Specifically, this means that if you find an auction with an extremely low bid using Auctioneer and bid on it right before the server goes down, you can pick up that item insanely cheap.
Hopefully, this method has netted me two stacks of Arcane Dust for the low, low price of 1 silver 10 copper. Since other realms may be suffering the same 48-hour downtime, this plan may help ease the pain of two days without WoW by knowing you're at least making some gold!
Edited to fix link.