Update: I took the client approach to initiate the login process, but actually did all the heavy lifting server-side by creating a customer MembershipProvider class and putting my custom code in there
Client: If "guid" is a variable in the querystring, immediately post the form
Server: MembershipProvider.ValidateUser: read the guid, call my other server, get back the username, and create the Membershipuser object based on that
There's got to be an easier way, though - surely there is some way in Default.aspx.cs to set the UserName and Password field and force a submit so that I can use the default MembershipProvider class. Darned if I could figure it out though.