CSR signature using elliptic curve
It should be pretty much the same. If you can sign a CSR with RSA you can sign with ECC. The exact same data is signed, what changes is:


The signature algorithm should be the right OID (e.g. ecdsaWithSHA256)
The signature value must be the ECDSA signature encoded as specified in SEC (i.e. an ASN.1 sequence of the two integers in the signature)
The public key value must a SubjectPublicKeyInfo structure with values specified in RFC5480: the algorithm field is an AlgorithmIdentifier whose algorithm field is the OID id-ecPublicKey and whose parameters field is the OID of the curve used. Finally, the subjectPublicKey field in SubjectPublicKeyInfo is the public key encoded as specified by SEC section 2.3.3.