If a game is capped at 720p, odds are it isn't going to play well at 1080p. Take Breath of the Wild, for example. It's capped at 720p on the Wii U, and the resolution is decreased even more when in areas with framerate issues, like Korok Forest. If you forced it to display at 720p in these areas, the frame rate becomes very problematic. At 1080p, it certainly wouldn't be playable.
Regardless, I am unaware of any way to force it to display at 1080p.