Dammit, you're right. I've actually got gyro-aim working by mapping the mouse input to the analog stick. It's not as smooth as I'd like, but it's working:
The problem is that moving Link with the same configuration requires flicking the trackpad forward constantly for character movement. It is possible to select absolute mouse movement in P64's menu, which lets you hold a direction on the trackpad like an analog stick and even get analog movement, but then the gyro aim is pretty wonky.
I suppose having the Steam pad output absolute movement on one trackpad and relative movement on the other might make this possible, but for now I think I'm going to have to let this one go.
This has been the biggest letdown of the controller so far- the pad itself is great and can do anything you want. The problem is on the software side of things- few programs or games actually allow you to fully utilize the controller's features. I'm hoping this thing takes off enough that developers start providing native Steam pad support in the future; one would think that emulators would be a big target for this controller since its configuration allows it to work with pretty much any game from any genre.