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
—

I think we need something similar to TARGET but with the POST binding.