Thank you! Can confirm linux-wiiu needs IOSU to init some stuff first, but the workaround was easy, albeit slightly clunky.
Successful boot:
: : : : files
fw.img = ISFSHax
minute.ini autoreload=false
ios.img = linux-wiiu
: : : : sequence
ISFSHax option 3
Hold B on Gamepad
fw_img_loader
ISFSHax...