All electric typewriters, and especially Selectrics, emit a discrete radio signal for each character typed, so you can see the problem. I am afraid manual typewriters are the only solution, and you'd have to control access to the ribbons too.
As a trained electronics and missile guidance technician, I’m not sure I buy into that theory. Maybe some of the later “self correcting” models, but the ones I used contained no electronics at all...purely mechanical.
But for arguments sake, it would be a chore to “hack” and office with 20 or 30 Selectrics going at the same time.
Either way, it would be a hellava lot more secure than wifi’s, the internet, etc, and the NSA would have set up shop outside thousands of offices to even detect such a minute signal.
Building a secure typewriter, even an electric, wouldn’t be that big a challenge.
1. Build a ribbon shredder into the unit.
2. If you want an electric, you could scramble the signals for the keys on a pseudo-random basis. You could use a number of things as a seed. One day, a certain signal will get you an ‘a’, the next day, hitting the ‘a’ key still gets you an ‘a’, but its just a different signal.
3. Go completely manual.
You can get efficient about things - you can record dictation and have a typewriter type it up for you. Do the dictation in a soundproof booth, an encryption key converts it to keystrokes, a screen allows you to proofread it in the Tempest hardened booth, and you walk the stick over to the typewriter.
Typewriter composes your memo, wipes the stick, shreds the ribbon.
Shove the stick in anything other than the destination typewriter and its circuits dissolve.