Hello,
good news to the scene: I've been developing a code handler for Wii U cheat codes and writing down an extensive code type documentation.
First of all: The code handler is not completely finished yet and support for more code types will be added.
In order to enable cheat codes you first need to run a
special version of the Kernel Exploit which mirrors the physical memory (range 00) to virtual memory (range A0). Now run pyGecko, which does not overwrite the mirroring (5.3.2 only).
Now start any game or channel. Once the game/channel is completely loaded into memory you can
run JGecko U. Click on connect and it will install my code handler into Wii U's memory. Now you can add cheats to the cheat manager and enable them. As of now you can have 0x2000 Bytes of cheat codes running. If this is not enough it can be increased anytime.
If you're interested in making your own cheat codes you can read the
Cafe Code Type Documentation.
If you want to use my code handler for your program you can get it
here.
Last but not least, Amibu is developing a
database for Wii U cheat codes.