What we need is a way for the OS to trick banking apps into thinking they are running on the platform they expect.

You cannot, the OS does not have that level of access. Attestation is anchored in a (typically) non-replaceable bootloader and trusted execution environment, both of which the OS does not have access to. A remote server can verify that the attestation chain is signed by a hardware-backed key and contains the verified boot status and verification key. If you would change this information, it would be detected by the remote server, since the signature would not be valid anymore.