yes it's possible.
Place the /apps/ folder on your USB root, for example : /apps/usbloader_gx/boot.dol
Then in the homebrew channel press 1 (or 2) on your Wiimote to display the settings and choose to display the /apps/ folder from SD or USB.
When you launch a homebrew, it's then that homebrew which determine if it's compatible with SD or USB, not the place where you launch it from.
USBLoaderGX doesn't load Wii ISO from SD card. but it can load GameCube ISO from SD card.