I implemented your first suggestion here and it worked great! I also managed to add extra fields, with a username that checked with the backend if it was available. Really smooth, thank you! Now I'm thinking if I could prefill an extra field. I want to have an "invite code" field so people don't have to wait for approval, and I want to send this in an email. I was thinking I could create a link like

/auth?code=foobar

and it would autofill.