This idea is simple on itself but more like an open question whether it could be used successfully.
It involves building haptic interface, possibly wore in a belt or similar piece of clothing that communicates morse back to its wearer. The simpler such device would use vibration but that most probably will (1) be very annoying to anybody nearby and (2) drain battery power very quickly. Other types of interfacing based on electricity or tapping might need to be considered.
A user most probably will have to be very concentrated to decode arriving messages but it allows for truly discrete communication.