It appears that the Wii U is capable of discretely downloading firmware updates in the background.
Multiple reports suggest that if you choose not to download the large launch-day update (a long process which has caused a stir amongst early adopters) the console will simply download the entire update in the background anyway.
This editor has experienced this first-hand today. My internet connection failed half way through the update, so the console abandoned the process and forwarded me to the Home menu. After an hour with Nintendo Land, I quit back to the Home menu at which point the console promoted me to apply the update, which had been seemingly fully downloaded and simply needed installing.
It seems that simply opting out of the initial update will bypass the need to leave the console downloading for up to an hour, and all you have to do to activate the background download is connect your Wii U to a wireless internet connection. Then enjoy an offline game.
NOTE: Do not attempt to cancel or disconnect your Wii U if you have already begun the update process - there have been multiple reports of system failures as a result.