For non-logged-in users, when they click Reply, they
currently see two textboxes where they can log in with
an existing nick and password.
Add textboxes for account creation to that same page.
Just the same form as normal account creation (complete
with HumanConf if enabled). Clicking "create" takes you
back to the comment-posting page with no more
account-creation boxes and lets you continue typing
your comment. But you have an email in your inbox with
a URL that will log you in.
The downer is that we then split between the anon
comment creation (typing lots of text into fields) and
the logging in (doesn't know about the text you typed
either before or after you clicked "create account"
button). Not sure of the best way to solve this.