Mayflash did make several Classic Controller to USB adapters which you might be able to use with HID to VPAD. They made three different models:
PC045
PC047
PC052
I've got a PC045, and I
do not recommend it for this purpose. The Classic Controller's input values are sort of weird, with analog stick values positioned at locations that aren't byte boundaries, and the PC045 passes them directly through. HID to VPAD expects each input to have its own byte, so while it works fine on anything that has proper USB HID drivers, the analog sticks can't be configured for HID to VPAD. You can still use Classic Controllers as digital-only pads (no analog sticks) this way, but it's not really worth it unless you only want to play classic games. On that stubject, PC045 is completely incompatible with nonstandard Classic Controllers like the NES and SNES Classic Mini pads, it only works with "normal" Classic Controllers like the original, the Pro, and the Hori Battle Pad (GameCube-style controller).
PC047 seems to be essentially a wireless version of PC045. You plug a Classic Controller into it, and a USB wireless receiver into the PC (or Wii U, in this case). I have zero experience with these ones, so I really don't know if it would work well with HID to VPAD.
PC052 is an updated version of the adapter. I've never used that one either, so once again I can't tell you if it works with HID to VPAD. According to some Google research, it does have improved compatibility, working with the NES and SNES Classic Mini controllers, at least. I think this one is probably your best hope as far as using Classic Controllers as Wii U GamePads, but I'm basing that only on the fact that it's upgraded over the PC045.