I have no familiarity with Reddit/Lesswrong codebase, but isn't this (r2/r2/models/subreddit.py) the only relevant place?

elif self == Subreddit._by_name(g.default_sr) and user.safe_karma >= g.karma_to_post:

So it's a matter of changing that g.karma_to_post (which apparently is a global configuration variable) into a subreddit's option (like the ones defines on top of the file).

(And, of course, applying that change to the database, which I have no idea about, but this also shouldn't be hard...)

ETA: Or, if I understand the code correctly, one could just change elif self.type == 'public': (a few lines above) to elif self.type == 'public' and user.safe_karma >= 1:, but it's a dirty hack.)