You could look at the problem this way: if your recovery is inside your rocket, you have to force your rocket to separate somehow. Currently a popular way of doing that is pressurizing the parachute bay with exploding gases (black powder) or cool CO2. This must be done at a very specific time, so you could use a timer (electronic or motor delay+charge) or an electronic sensor. It seems you want to find a solution that is 1) not integrated into the motor and 2) not electronically detected. If you want to use a mechanical device, first think about the conditions/state (altitude, velocity, acceleration, orientation, pressure, magnetic field, past states, etc.) of the point at which you want to eject, during the flight, on the pad, and during assembly. Determine how your ejection state is unique from any other state (you wouldn't want to eject in any other state). Once you have this problem figured out, you can start coming up with devices that will only work in a single state.