I changed the OSDynLoad_FindExport return type since it's wrong in libwiiu, it returns an int.
I didn't change anything at wupinstaller.com.
You might want to compile the code with -Os
Aha, thanks for the info. @wj44 was kind enough to recompile the source into a working binary: http://wj44.ml/wupinstaller/code532.bin
With this (after running the kexploit first), I was able to spoof my 5.4.0U console to 5.5.1U! One thing that differ from 5.3.2 though, after being sent to the Home button menu, I pressed the home button and then got a black screen with Error: 0x00000000 text and I had to hard power down the system. However, when I booted back up, I was spoofed to 5.5.1! So the question for other updates is, should I sit on the Home button menu and let it install updates in the background there, or will it install them in the background on the Error screen?