Redmine

Popups (inline or alerts) are generically dismissed by hitting the "ok, i've read it" button within seconds after it has been shown. Even the ones with scrolls that you need to pull down also misses it's mark.

My personally opinion is that this will not solve the problem. But that's ofcourse just my $.02.

I think that most of users that want to check issue posting process expect that it will take a small amount of time, so... Maybe just make new user registration process more difficult? For example before first post into real readmine database all new users should wait some time (24h)? Of course, user should be mentioned about this and alternative registration link to test database (where isn't such restriction) should be provided :)
Or maybe first post into real readmine database should be able only after user posted at least one issue into test database? :)