I had a really similar idea last weekend and hacked something together that does this. Now when friends come over, they can just scan a QR code and it automatically logs them into the guest account and redirects them to a limited guest dashboard. I just wrote up a blog post on how to set it up. The Completely Frictionless Way to Let Guests Use Home Assistant - Nathan Orick
Edit: Quite a few folks were using this, so to make it easier, I turned it into an add-on: