SAML: APM as Service Provider (SP) role - is it possible to do SSO credential mapping to the backend server?

Why use SAML if your backend already knows the username and the password........

The purpose is to use the F5 SP as the landing/gateway and use it as an auth broker as multiple backend applications with varying requirements - like OWA and custom IIS apps with NTLM. The F5 SP should be posting the creds to the backend app here with NTLM/forms auth. With the following iRule, I can get the user identity and any attribute in the SAML but not password. I wonder if F5 is stripping/blocking the key word "password" in the attribute or something is wrong with my b64decode.

Any help is highly appreciated.

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when ACCESS_ACL_ALLOWED {

        set username [ACCESS::session data get session.saml.last.identity]
        set password [b64decode [ACCESS::session data get session.saml.last.attr.name.password]] 
        log local0. "Username -$username, Password - $password"       
}
when ACCESS_SESSION_STARTED {
    if { [ info exists username ] } {
        ACCESS::session data set session.logon.last.username $username
    }
        if { [info exists password] } {
        ACCESS::session data set session.logon.last.password $password
    }
}