After doing a basic Google search (can i turn off wii u while it is updating)
It would appear that some users have in the past reported issues when the power has been lost to the console for some large firmware updates,
Shutting Off Wii U During Massive Update Bricks Console
For instances other than firmware updates this site Wii U Forums indicates that the power light turns orange when the console is connected to the internet checking for updates/new content
It's sleepmode downloads.
The reason your Wii U sometimes has an orange light is because it's downloaded an update/patch/demo/game for you to use/play next time you
get back on it, for example Nintendo has made it so it automatically
downloaded Wii Sports Club into every Wii U.
It (by default) checks for internet connectivity once every 6 hours,
and if there are updates, will download them.
It also states on that page that you can turn off standby updates in your settings therefore if you are worried about possible damage why not turn off the standby updates as then when you power off your console using the button on the console any downloads will stop and you can safely turn off the power strip.
guidance on how to do this can be found on Nintendo's website
Obviously if you have allowed an update which warns you not to turn off the console until the update has finished then you will need to leave the console powered up.