How would the RP indicate what it wanted? As a (newly defined) required claim URI? In this case the RP is not simply asking for a piece of static identity data that is the same for all RPs, e.g. email address etc. It's asking for 'the username & password that the user previously stored against my endpoint'.
In this sense, the username & password are just like the existing 'private personal identifier' in Cardspace (or a pseudonym shared between a SAML IDP and SP).
Is it just another claim?
<OBJECT type="application/x-informationCard" name="xmlToken">
<PARAM Name="RequiredClaims" value=
Presumably, current selectors would not know to process this as it would a PPID, and not an email address.