I think the original idea (or at least the one that DARPA is researching) is that this tech will be only used on government computers, so, assuming they all use

I think the original idea (or at least the one that DARPA is researching) is that this tech will be only used on government computers, so, assuming they all use the same computer, that should eliminate some of the problem. For other uses, you're right, this would be impractical, because typing on a physical keyboard is obviously different than a touch-screen phone keyboard.