Hi
I am also interested by a solution, this is obviously doable, (there is even a module that log you with only the username).
Maybe you can try to see the java action used during a normal login process, and reuse that java action in your microflow.
You also said that you already check the user pass combo, so you just need to login the user, if you did not find a solution you may want to use the module that log you with only the username (it is called autologin), but I would strongly recommand you to not allowed anonymous user to use it (in the microfow, put the allowed to none)