Something similar was asked here: New "Authentication Challenge" Feature in Perspective - #2
We created ticket IGN-6703 to track a new feature to be able to supply a user "hint" to the IdP to influence which user is attempting to login (can be used to pre-fill the username field on the login form, or perhaps for some IdPs which support multiple user logins per session, it can select that account automatically). I'll link this forum thread to that ticket.
The ticket is still incubating, so it will not be ready to work on any time soon...