Can you self sign other people packages with Charles? That's crazy!
Now on a closed device it can be very difficult to add root certificates to the store, but it's often possible.
For instance when the PS3 firmware was first cracked I took the opportunity to flash mine with a firmware I made that contained my root authority certificate. Then I wrote a python program to intercept and MITM all the traffic.
Result? I found out that on boot your PS3 tells Sony every game (or other thing) you run on the ps3, what times you run them and how long for.