Can a 220-240v WiiU adapter be used at 110v?

Disclaimer: The following answer is purely speculative, and intended for informational purposes only. Should you choose to try and use your WiiU power adapter at a voltage it is not rated for, I take no responsibility for the consequences (which, at the very least, likely include voiding the warranty, and might include damaging your adapter and/or your WiiU, or worse).

That disclaimer out of the way, let me note that that most modern switched-mode power adapters are designed for worldwide operation, i.e. to handle (at least) any input voltage from 110 VAC to 240 VAC at anywhere from 50 to 60 Hz. The reason for this is simply that it's easy to do, and it generally costs less to use a single standard worldwide adapter design for all regions than to design and build different power adapters for different parts of the world.

That said, usually such universal power adapters are also labeled with something like "input: 110–240 VAC". The fact that yours isn't might be due to several possible reasons:

• The adapter might not actually be universal, and might break or just not work properly if given the wrong input voltage.

• The adapter might be designed to handle any input voltage, but the manufacturer might use more restrictive labeling to cut down on testing and certification costs (the specific adapter-and-plug combination might've been intended only for 220–240 V markets, and might not have been specifically tested at 110 V) or warranty returns ("Oh, you used the adapter at the wrong voltage? Sorry, no warranty for you.").

• The restrictive labeling could simply be a half-hearted attempt to limit "gray market" imports, and thus allow more variation in regional pricing (market segmentation).

• Some combination of the above.

As you note, a quick Google search does turn up several sources claiming (and demonstrating) that, despite not being labeled as such, WiiU power adapters do work with any voltage. Granted, those sources are mostly about using the 110 V adapter at 220–240 V, rather than vice versa, but it really makes no sense for them to use a universal 110–240 V capable circuit for the "110 V" adapters, and not use the same circuit for all adapter types.

In any case, whereas using a non-universal power adapter at too high voltage might damage the adapter (or cause it to deliver overvoltage, damaging whatever it's supplying power for), using too low input voltage should, at worst, only cause the adapter to supply insufficient output voltage and/or current. This might cause the connected device (your WiiU) to fail to start up, or possibly to "brown out" and crash at high load, but it's rather unlikely to cause any permanent damage. (But do remember the disclaimer above!)

If you happen to have a multimeter lying around, one test you could do (at your own risk!) would be to plug the adapter in without connecting it to your Wii, and measure the voltage it delivers. Of course, this simple test doesn't really prove that your adapter can still maintain that voltage even under high current draw, but it should at least provide some reassurance about your power adapter's voltage tolerance, before you actually try using your WiiU with it.