Thanks, Philip, for some reason it didn't work for SP-Initiated SAML SSO when I tested it but I will give it a try again out of curiosity. It would not really meet the requirements of our security posture but it could be a good workaround in some other scenarios.
To be honest, I could not have figured out that the "username" SAML attribute value could be anything but an email address, as it happens for locally defined accounts.
Regards,
JC