
The obvious solution, then...
... would seem to be to turn off accelerometers, gyroscopes, and (optionally) cameras after the first keypress on a device's (virtual or physical) keyboard, then don't let the sensors reactivate for, say, the first 500ms to 2 seconds after the last-registered keypress.
Since passwords, social security numbers, and other ID-oriented strings are likely to be entered quickly, because of the advantage of "muscle memory," it's improbable the device would encounter long delays between the keypresses used to enter often-repeated data. Therefore, a 1/2-second to 2-second delay in re-enabling the sensors should suffice to reduce the input inference score to 5 percent or less, depending on the length of the string being entered.