extending SubjectLocality

A SAML authentication statement has a SubjectLocality element that
carries an (optional) IP address.

SAML V1.1:
AuthenticationStatement/SubjectLocality/@IPAddress

SAML V2.0:
AuthnStatement/SubjectLocality/@Address

Suppose an IdP were to map this address to a two-letter country code
(e.g., US).  What is the best way to carry this two-letter country
code in SAML, in an authentication statement or an attribute
statement?

Thanks,
Tom