Capcom USA has confirmed to siliconera that you won't be able to play Monster Hunter 3 Ultimate on the Wii U gamepad screen.
You will, however, be able to rearrange the UI on the Gamepad screen to suit your playing style, so that's a plus.
It's unfortunate that off TV play is not supported, but I guess they feel the extra controls on the screen are too important to give up? Who knows. Maybe they are trying to drive sales of the 3DS version as well, so to play it on a handheld you need the 3DS. But that's just cynical me talking.