Answer
This is confirmed as a limitation on the SCIM protocol side, not something IFS can address — there's no mechanism to override the default Identity-creation process during SCIM provisioning, and no SCIM attribute exists for mapping a custom identity value; if a custom Identity value is a hard requirement, it needs to be handled outside the SCIM provisioning flow itself (for example, provisioning the account through a different mechanism, or adjusting the identity afterward) rather than expecting SCIM configuration to support it directly.