I think for quick booting into Wii mode by holding B...A small test via HDMI cable is required. The HDMI cable must be hooked up to your TV and Wii U and the tv must be powered. Haven't tried with AV...
But can anyone confirm that? Try doing it with without an HDMI cable...
You don't need a sensor bar at all though for the quick Wii mode boot, I've tested several times without it plugged in. You'll need it if your on the Wii U menu though.