The idea is genial - relatively simple and very efficient. And it can serve as a basis to implement additional features.
In my thesis "Detection and countermeasures of attacks on smartphones" I described a possible solution with a so called Turbo SIM. Analyzing the communication of the SIM card can also be done with a WiFi- or NFC (Waver) Turbo SIM. That means you don’t have to connected the „battery case“ with the SIM Card slot. If people think this device looks to too obvious, it would be possible to place it inside a 2,5 HDD case for example.
I am looking forward to this project - big respect!