Voultar has just released a DIY kit that allows you to unbrick Wii U’s with failed NAND memory. This is a two-piece solution that’s designed for experts already comfortable modding and backup / restoring NAND memory on Wii U consoles. More info after the link:
The first piece called the NAND-AID allows you to use a MicroSD card to replace the internal memory without removing or “hot airing” the old surface mount NAND package. The NAND-AID also allows you to manually back up the failing memory off of the original NAND when used with the SD Tool…
…and the SD tool is a NAND dumper/writer jig that allows you to take advantage of the fact that the fast majority of SD-Card readers have the capability of reading and writing to eMMC/NAND memory! This tool isn’t just for the Wii-U but can be used with any device that uses an eMMC NAND, such as the 3DS and specific Xbox 360 models!
The reason these should be considered expert-only, is there’s no support and documentation – These are designed specifically for people who already understand the complicated procedures necessary, but want a tool to make it easier.