[keycloak-user] Setting NameID to Unspecified

Hi all, I'm working with a particular IdP client, and they have requested that I set the NameID field to an attribute on the user that is neither username or email, and that it must be in the "unspecified" format. I've been trying a bunch of different configuration options to get it work, but none seem to do what I need it to do. I know about "saml.persistent.name.id.for.$clientId" on a user, and I've been trying variations on that. Does anyone have any guidance on how to have a attribute of the user be populated in the NameID field, with a format of "unspecified"? Thanks, Ron