Hi,
I haven't tried this so I am guessing here. BU I would try to implement standard service provide initiated SSO with front-channel communication. In OASIS spec in section 5.1.4 the fifth step is to call Assertion Consumer Service using HTTP POST. This is implemented in SICF service saml2. So I would try to set up your IdP to call this service and debug this service to see what is happening.
I would also check Automatic User Account Creation and Update using SAML 2.0 in AS ABAP - Security and Identity Management - SCN Wiki This scenario is also IdP initiated but it also creates a new user. So it should be possible to use it as a template and omit user creation step.
Cheers