Then let me make it as easy as possible:

--- a/r2/r2/models/subreddit.py
+++ b/r2/r2/models/subreddit.py
@@ -173,6 +173,8 @@ class Subreddit(Thing, Printable):
return True
elif self.is_banned(user):
return False
+ elif self == Subreddit._by_name('discussion') and user.safe_karma < 5:
+ return False
elif self.type == 'public':
return True
elif self.is_moderator(user) or self.is_contributor(user):

Note that this is a bit of a hack; the right thing to do is to replace the karma_to_post global variable with a member of Subreddit, and make some UI for adjusting it. One side-effect of doing it this way is that the message users get when they don't have enough karma (from r2/r2/templates/newlink.html:81) always says they need 20, regardless of which section they tried to post to.