[shinigami]

Subject Public Key Info is just an Algorithm Identifier and the public key. The Algorithm Identifier is an OID and the parameters (ECParameters when using EC keys). It's these parameters that can contain the custom EC domain parameters.

The certificate signature is preceded by another Algorithm Identifier that specifies the signature algorithm (and the parameters), and so it seems that Microsoft is using this value instead of the parameters in the signer certificate Subject Public Key Info?