If that usb loader supports gamecube remotes, you can use that to access the homebrew channel.
I'm not too sure how priiloader works, I use bootmii myself. But if I didn't want bootmii auto starting when the wii does, I'd just rename the bootmii folder to bootmii-. If Priiloader has a folder on the SD card like that, then maybe you can do the same.
Or just take the SD card out of the wii