Accessing raw 5V inside the phone to charge an external battery

We are trying to make a customised iPhone 7. For this project, we will have an extra LiPo battery mounted on the back of the phone as well as some special decorative LEDs and PCBs, and these will be enclosed by a custom-designed shell. The shell will be attached onto the back of the phone permanently. The intention is not to make a phone case but to make an expanded iPhone by attaching components on the back permanently.

Now the challenge is charging this extra battery and the genuine battery at the same time. Ideally we want to achieve charging two batteries together through one lightning connector. This means, we make a small hole in the back aluminium panel of the iPhone and let two wires from our battery charger inside the phone, then connect these wires to a point on iPhone's PCB which provides ideally unregulated raw power from the USB cable through the lightning connector.

I have opened the phone and found two testing points, near the battery connector, which provided 4.2V but was not sure if these points could be the ones available for this purpose. I could see several capacitors and resistors near the lightning connector but all covered by something like resin for protection, and have not tried to unwrap this cover.

Can anyone on this discussion board help with this investigation? We are aiming to commercialise this project in the near future as luxurious customised phone.

I understand the risk of doing this and have already had a conversation with Apple. Basically this is not illegal or against their policy but the customers of this customised phone will lose the warranty.