Too bad there's no way to bypass that with HDMI cables, it's kind of annoying and counterproductive to not be able to use off-screen play with HDMI if the TV isn't on. Component is perfectly fine however.
I believe you can go in to system settings and turn off the HDMI connection.....however if you don't remember to change it back the next person has no idea why the wii U is not working.
Its simpler, yet still annoying, to have whoever is watching TV switch the input for a few seconds. With DVR I can pause live TV to do this and don't have to wait for commercial but it's still inconvenient to me and whoever is watching tv.