First, to make things clear, it's not available yet but they are working on it.
Ardi just updated the forwarder's source (the new forwarder installer/wad is not out yet) and added support for loading the boot.dol/boot.elf from the USB fat partition
Here is the changelog:
makes the forwarder fit for loading the "USB Loader GX" from USB-Device
dimok also had also already stopped by the forum and here is what he said:
... Ardi made that thing work. He changed stuff also inside libfat (which will come soon) to make it work. The USB stuff was implemented long time ago by me but it didn't work since the ports for it were blocked by IOS249/222 (at least for the USB Port in which the WBFS HDD is). Thats why we didn't use it and only commented that code out. Now that ardi made it work and is testing it in fully we just need to uncomment the code and we can use it.
Its going to be searching for the FIRST available FAT/FAT32 Partition on your USB Device and mount that. You will than have the option to just change the path or whatver you use to "USB:/" instead of "SD:/" and it should be working.