\The whole, "Can't use an SD card over 2GB" is an oversimplification of what's actually going on. SD cards by default are formatted to FAT16. Original SD cards were designed with 2GB as their max limit... Then a new standard came out with a 32GB limit called SDHC. (High-capacity). These cards are formatted to FAT32.
The size a particular card is formatted to is not something that would usually affect compatibility unless the hardware specifically has a cap. Generally, it's a matter of whether the format is supported or not. The Wii and Wii U are capable of reading SDHC cards for other things, so this means we've narrowed down the actually reason to two possibilities (that I can think of):
1) There is a vulnerability in SD cards that was fixed with SDHC cards, and the hacks make use of this exploit.
2) The hacks only work on FAT16 formatting.
This is where my knowledge stops. Without knowing the technical details on how the hack works, I can't really say exactly why it only works with one and not the other. However, if the second option is a possibility... That means in theory you could find a third party tool to reformat your SDHC card to FAT16 and give it a try. (Google it; there are tools out there.) I'd be curious to know your results, as I haven't heard of anyone attempting this.