That sounds laborious enough to set up that I don't think I'd consider it a serious vulnerability, but I'm having a lot of trouble coming up with a scenario where it'd be worth the trouble. There are situations where I might benignly want to write an anonymous top-level post, but relatively few that absolutely have to be outside the discussion section, and essentially none that also have to be done on zero notice.
At least aside from perpetrating some kind of drama, and I hardly need to emphasize why that's a bad idea.
It's not about being able to post anonymously on zero notice, but about removing the temptation to skip making an anonymous account in borderline cases by converting it to a sunk cost.
The discussion section does change the calculation somewhat, but getting 20 karma on a clean account is not much more effort than getting 2 karma. (Someone explained the strategy elsewhere on this thread.)
I just noticed that calcsam, who just posted two top posts in the main section of the site, only has the 100 karma that he has, so far, gained from those posts.
I don't object to those posts being there, but how did he do that?
Edit: Question answered; Eliezer mucked around with the karma system to make this possible in this specific case.