it sounds like you want two separate things:
- for the firmware to be inaccessible to endusers
- for a paid subscription service that jolla promises to reflash it for you
there’s nothing wrong with #2, except that it’s strange to want to pay money for it, but you could accomplish #1 with an optional firmware locking password, like what Bohdan is suggesting. then it would actually be opt-in.
i don’t understand what you want this for, except to spite thieves. it wont prevent the theft, and if what you’re worried about is your data, then you can just use that cryptographically secure password for data encryption instead of firmware locking…