I’ve recently had a similar thought. However after looking into it, I’m not sure whether this is feasible anymore.
[Android Compatability Definition (pdf, p 48)] lists a requirement which might be hard to meet with an aftermarket fingerprint sensor in the back cover:
MUST have a hardware-backed keystore implementation, and perform the fingerprint matching in a Trusted Execution Environment (TEE) or on a chip with a secure channel to the TEE.
I’m not an expert though and might be completely wrong about this: Maybe it is possible.