This is a proposal to add a handler like this in Shibboleth:
<Handler type="AttributeQuery" Location="/AttributeQuery" acl="127.0.0.1"/>
This handler could be accessed using more or less the same parameters as the arguments of the resolvertest binary. E.g. /Shibboleth.sso/AttributeQuery?nameId=1234asdd&entityId=https://example.org/idp/shibboleth&protocol=SAML2&nameIdFormat=urn:oasis:names:tc:SAML:2.0:nameid-format:persistent
The SP - provided the acl allows ist - then would make an attribute query to the given Identity Provider and return the attributes (similar format like for the Session handler) or an error message.
Such a handler would allow to easier check whether a user still has an account (orphan checking) at an Identity Provider and it also would provide a useful mean to update a user's data without any interaction by the user himself.
Using the resolvertest currently is not a suitable option because it is very slow (>10s per request beacause it loads the whole config) and because it probably was not meant for this job.