I wonder if playing the typing sound constantly could help. Not an abstract sound, but recording of your actual typing on this particular keyboard, mixed to play some realistic-sounding phrases / sequences. It should pause for a split second to let your actual keystrokes mix in. That would be really hard to decipher, or to correlate your typing with whatever other events (time to enter a password).
Better yet, play some white noise around you. I heard that it's actually done sometimes at really important meetings.
If you're not such a VIP, just type important things only on your phone; touch screens don't produce enough sound, hopefully.