Creating a Custom Title Screen
Tools Required:
Of course, make sure you have a NSMBU rom obtained from your Wii U console. A guide to dumping games can be found here.
Importing a custom logo
First you're going to find the Title.szs file. (This contains the asset used for the the title screen logo) So go to this directory in your NSMBU rom: content/common/layout..
You should now see these files
Next, open Title.szs with Switch Toolbox.
Now locate the file inside of the szs. The image is located at lyt_root/timg/wiiMario_Title_logo_00^l.bflim
Now open this bflim by double clicking the it in toolbox.
This new window will pop up.
Now in the BFLIM editor, click "File" -> "Replace".