Have to agree, the mech design for the matrix movies was *utterly* retarded! - Not even a bit of wire mesh around the pilot!?%$
But to get back on topic, you can expect most levels to use either "you are leaving the mission area" zones and/or physical boundries (mountains, rivers, etc.) around the play area..
Cryengine's gravity volumes do have a parameter for falloff, so in theory you could make asteroid levels like you described, and I have been playing with some concepts, but in practise gravity seems to be a bit buggy if you want anything other than earth-normal or zero-g...