there is a way to dump to HDD, but it's beta and could not work correctly.
the trick is to use (abuse) a bug in the latest beta of Mocha CFW.
The beta tried to add compatibility to USB, but it's incorrectly mounting the USB first before the SD card internally, and the console expect the first mounted device to be the SD card.
as a result, the USB is seen as SD card by all other homebrew.
So, what you need :
1. put Mocha beta 0.2 on your SD.
2. launch it (if you use CBHC, access the menu to launch mocha instead) from homebrew launcher or from haxchi.
3. connect a FAT32 HDD
4. launch any homebrew using SD card, everything will be read from USB.
Attention, you need to put your homebrew on USB too !
the USB became your SD card ! all the "sd:/wiiu/apps" need to be located on "usb:/wiiu/apps" or you won't find them in homebrew launcher.
remember, it's beta ! not working fine with everything.