The first time you try to unlock the PC this way, Steam will prompt for elevated permissions to be able to do so.

You can of course also set up the user to be automatically logged in.

In my experience it's best to have Steam launch via the Task Scheduler with elevated permissions so that you can approve UAC requests, etc.

However, yes it's worth having a keyboard connected (even if it's just one of those tiny wireless touchpad+keyboard combos) in case things go wrong.