Not sure if anyone's hit on this yet, but I found an unused parachute container model in the IMG file. It appears to be modeled after an actual rig, and looks 100x more realistic than the "rockstar backpack".

This model and texture file are only for the container - retexturing the canopy will be left to you On a side note, I also found unused animations related to the canopy. These too, are much more life like, and I wonder if anyone cares to get them back in the game?