The Mocha mod is actually for using FAT32 instead of Wii U formatted drives (which as said doesn't work perfectly), but as a side effect of this the USB drive will take the place of the SD card as well, so you can use it for anything you would normally need a SD card for, homebrew will be loaded from it etc.
Here is a compiled version of it:
https://gbatemp.net/attachments/mocha-cfw-zip.77927/
Simply by inserting a FAT32 USB drive (with needed homebrew such as WUPInstaller on it since all homebrew ran from this Mocha version will be loaded from the USB) while running this version should allow you to use the USB as if it were a SD card. You'll have to install titles to NAND though, if you want them installed on a USB you have to switch back to regular Mocha or OFW to move them onto a Wii U-formatted USB afterwards.