Unsolicited IDP initiated Auth via POST binding

We have a service provider that want to use IDP initiated auth. We must POST an assertion to their ACS without ever being solicited with an AuthN request from them.

IRC snippet---
asaldhan dminnich: if you add the TARGET attribute, then it is idp initiated with HTTP redirect binding to SP

asaldhan dminnich: if u just add SP urls to the hosted page, then you get the SP sending AUTHN request and getting back a SAML response right away
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I think we need something similar to TARGET but with the POST binding.