The Agentry SAP products support that exact scenario today. The login request is passed to Agentry (no authentication at SMP level typically) and Agentry connects to the portal to authenticate the user / pwd from the client. The portal in turn generates a login ticket and returns it to Agentry. That login ticket is then used when establishing the connection between Agentry and SAP. There is a step in there where the user mapping occurs that I am not 100% sure about (haven't got that working in my test environment yet) but is it supported.
--Bill
