Sign language? Can't be overhead, and you can use your body to block your communications so they can't be seen. (You can *whisper* to someone in sign by making your signs really small so that only they can see).

Alternatively, morse code using touch. Morse code already exists, you wouldn't have to spend too much time developing it. You might not even have to ever broach the idea of using it out loud - everyone knows SOS. if someone starting tapping that on my knee in an orwellian world i would go out and learn the rest of morse code to find out what else they had to say. Light taps can be easily obscured by clothes/blankets/screens.

I think there would be an element of risk no matter what system you choose. The consciousness of this risk can only improve your story by adding tension.