No need to vent to outside if you use a dry cell, perfect example is the factory setup on later model skylines (later r33, r34 etc).
Dry cell batteries still emit hydrogen, just no where near as much as the wet cell brothers.
Just a note, if you are going to ground the earth in the boot, make sure to remove paint of contact surface, and check theres good continuity with a multimeter.
Personally I'd run both cables back to the engine bay, but it'll be a weight penalty for those that watch their weight