I actually was able to use USB instruments by playing vWii games through USB Loader GX and using a separate HDD in Fat32 format. Wii games HDD goes on USB 0 (top back) and USB hub and instruments, including Wii U games HDD on USB 1(bottom back).
I am using the standar cIOS 56, dsx beta 52 on slot 249.
I am using the standar cIOS 56, dsx beta 52 on slot 249.