Registration after creating a post

How would you suggest implementing the following lazy registration process?

1. Non-authenticated user completes a form that would generate a post (custom)

2.a Once the user submits this form, the post is immediately public and its author is a generic "Guest" account. (Ideally, a unique account is auto-generated following some convention like "Guest_#")

2.b The user is asked to either log in, or to register a new account by entering email and password (on front-end, not going to the Dashboard). Ideally, they can do this any time as long as the session or cookie has not expired.

3. After the user completes login or registration, the post's author changes to this account

We are just setting things up, so not attached to any particular plugin, theme, or framework. Thanks in advance!