I'm on 10.1, and since eShop spoofing is broken, I cannot download badge arcade. Do you need to be on the latest firmware, and have to have played badge arcade atleast once to use custom badges?
The only thing stopping us from this being possible afaik is knowing the hex ID of your NNID. it's the 4-byte value at 0x1C in "BadgeMngFile.dat". I'm not sure if this 4-byte value can be found in other ways with homebrew
Edit: I think I just found it... 0x2ED2C in the home menu layout save! going to test now
It seems the value only appears there aswell if a layout with badges was saved, and that would require running the badge arcade...