There are jump points under the hood. Use those first to try and jump the vehicle long enough to open the hatch. Seeing issues people have had doing this in the past, you may want to verify you have a solid connection especially on the grounding post (the long grey 'bolt' sticking up in the engine bay on the drivers side near the headlight). Also may need to give it some time if the battery has been seriously discharged to give it enough juice for the appropriate modules to wake up. The BCM is in charge of the locking system and the latching mechanism in the hatch. If the door locks work and interior lights are lit, it should be enough to open the hatch.
If all this fails, you'll unfortunately need to climb over the back seat and figure out how to access the battery that way which will be a HUGE paint in the ass. Try the above options first.