I'm not sure if this is possible, but if you could slide something underneath the plastic panel, just above the latch, there is a plastic slider that locks and unlocks. You have to slide it towards the driver's side.
It is probably too high behind the panel to get to though, so you will probably be better off trying to get the plastic panel off and unlocking it by hand. There are a few screws, depending on year, and christmas tree clips holding it on. It might be a bit of a bitch since it probably wraps around the hatch opening a bit.
Or you can just remove the broken key and have another one made with the broken piece.
Or you can pull the whole lock cylinder out and take it to a blacksmith to have a key made. That's what I did for mine since I didn't have a key for the doors when I bought the Jeep. But the lock cylinder got really sloppy after they took it apart and put it back together. Eventually, I couldn't get the key in and had to unlock the passenger's door, then climb in and unlock my driver's door. If you go this route, I would suggest using the passenger's side lock cylinder to have a key made.
God luck.