Uhh, how did you come up with that number? It's only 4 question marks, 4 bytes. There are 16 possible hex values each of those bytes could be. 16x16x16x16=65536 possible variations for the end of that key.
But also, so far all of the WiiU keys have been 16 bytes long... With 4 question marks that would make this key 18 bytes, which seems weird and unlikely. I think he might have just added those question marks to confuse us. In reality it's likely only two unknown bytes, and thus only 256 possible variations.
But either way, brute forcing even 65536 values is nothing.