I'm not sure if there is a known method for changing boot images for WUP Wii U or Eshop files but if someone does i would like to know myself. The only thing i can tell you is how to change boot images for Loadiine and VC games (which includes WUP install for VC with the right tool).
First get the program Paint.NET. You should be able to export the image you want to use to tga but if problems occur there are others means to convert the images (google). You need to resize your boot image to 1280x720 and rename it bootTvTex.tga. If you want to do the gamepad as well then make a copy of your image and resize it to 854x480 and rename it bootDrcTex.tga. When you are saving the images it will show you some settings, make sure image compression is turned OFF or the Wii U will not recognize them. Lastly get the program TgaVerify (you will need Python 2.7 installed for it to work). Create a folder named meta and insert the program and images into it (image names are case sensitive), it will scan them and should tell you if there are any problems. That is it.
I was going to share a link for a batch injection tool for WUP that greatly simplifies VC injection but im not sure if its okay to share here. You can PM me if you're interested.