I loaded TS through Steam, so I could see how the game plays with the Steam Controller. Just a quick controller remap later, with the 2 touchpads mapped to joysticks, and I was up and running with some very responsive controls.
I'd love to see this with official Steam Controller touchpad support, i.e. the position of your thumbs on the pads are mapped directly to the vertical position of either side of the bar (with some additional smoothing obvs, since thumbs are not perfectly still) :)
We've experimented with similar control schemes no mobile, but unfortunately they don't quite gel with how the input in the game works. One of the main movement mechanics is holding up at the same time on both sides of the vine, and if the sides are mapped directly to finger input then you get a really awkward rowing motion when trying to move up simultaneously (if I'm reading the idea correctly)
Hmm, to combat the rowing I'd try setting an upper limit on the touchpad/device, and when both thumbs are above that, level out the vine and have it move upward (maybe?). I dunno, it makes sense in my head, but you're the ones who've actually experimented with the controls so I trust ya :)
You need to add TumbleSeed to Steam as a non-Steam game, launch the game with your Steam controller from Big Picture, and then press the Steam button on the Steam Controller to access the overlay.
Fun note: I keep forgetting that you don't need to be in Big Picture mode to get at controller settings (still recommended, though). You can also right click on the game in normal Steam.