Thankfully, the hard work has been done by ABC and Calin Culianu, who ported the entire library into Electron Cash. Now the question is how to actually implement?
Adding Schnorr to Electron Cash
Currently, the sign() function in lib/transaction.py creates a signature using ECDSA-related objects like MySigningKey class from bitcoin.py, and sign_digest_deterministic, which I believe operates on a private key object returned from the ecdsa library.
Instead of this, the sign() function should directly call into secp256k1/src/modules/schnorr/secp256k1_schnorr_sign using the message and private key.