I'm assuming this is meant to be in addition to instead of replacing the built-in keyboard. At a bare minimum such a thing must allow significantly better typing speed than the built-in keyboard or else there isn't a real point in having it, no matter how large it is. From what I'm reading in reviews, it's difficult to raise above 40-50 words per minute even after acclimating to the device, and the vendors recommend looking at the keyboard while you type. It's not hard to see why this would be the case, normally we use some degree of tactile hints to keep aligned. And even acclimating to this system sounds laborious, at least for me. I remember when I broke my left wrist; not only did my typing speed drop from > 100 wpm to < 30, but I now couldn't touch type whatsoever and had to look at the keyboard. Despite "knowing" where the keys are it's so ingrained in muscle memory it's not something I can easily adapt to a different pattern.
I think it'd be a no-go for other reasons anyway. The standalone devices out now, while reasonably small for what they are, are still much larger than the acceptable footprint available for adding to Pyra. The battery life isn't very good (< 3 hours) which suggests a non-negligible power consumption overhead. They're also expensive (> $150), and while I'm sure the BOM cost is just a fraction of that it'd really have to be under $5 to even be a serious consideration here.
