ERC1271 Validation on Gnosis Safe from a Marketplace

Question

I would like to create listings on marketplaces like LooksRare, where they use off-chain (signature based) maker orders, but I am not sure how to do this with a Safe.

I understand the role of the CompatibilityFallbackHandler, and the SignMessageLib contracts, and I have been able to add signed messages to my safe... what I do not get, is how the CompatibilityFallbackHandler will ever return MAGIC_VALUE for a 1271 signature.

// Marketplaces call IERC1271 like this
IERC1271(signer).isValidSignature(digest, abi.encodePacked(r, s, v))

// Compatibility fallback is implemented
isValidSignature(bytes memory _data, bytes memory _signature) {
...
      // How can I ensure  _signature.length is 0 here?
      if (_signature.length == 0) {
            require(safe.signedMessages(messageHash) != 0, "Hash not approved");
        } 
...
}

UPDATE: For marketplaces who call isValidSignature like above the alternate method with args (bytes32, bytes) overloads the other function so that is fine. Still wondering how I should setup the ISignatureValidator with a custom isValidSignature. I will just create a similar FallbackHandler contract for now.