I am currently implementing an ISO20022 integration at a large financial institution. The way these generally work is that whatever network you're connecting with will require a certain standard number of an ISO20022 message. When you send the generated messages, they must be valid as per the specific schema in question. If not, they will be rejected. I'm speaking specifically about payment networks, not consumer level payments.

Generating classes from the XSD's is fairly trivial in most languages. The hard part is being able to read the XSD's and then being able to create the ISO20022 message with all of the required elements (given all the possible combinations of valid elements).

I guess creating a code-time library where it was not possible to create an ISO20022 message that invalid would be interesting. But being able to create invalid ISO20022 is fairly easy to do for free.