wfz on 20 November 2012
Yes, you can install and play games off of an external USB HDD. There appears to be an issue with certain USB 3.0 HDDs, so your best bet is using a USB 2.0 HDD or going off of Nintendo's support site for recommended HDDs (although it is currently pitifully lacking).
http://www.nintendo.com/consumer/systems/wiiu/en_na/external_usb_devices.jsp
