Yeh - (Unless anybody knows differently) I don't think such a thing exists. If you're writing this as a plugin (you should be) you'll need to implement your own web service to process the username/password.
My first thought would be to have a look at some of the existing custom themes to see if anybody has done something similar.
My first thought would be to have a look at some of the existing custom themes to see if anybody has done something similar.