Mechanisms to prevent eternal September are to assimilate new members more rapidly

There's a limit to how fast this can be done. That's, essentially, why something additional is needed.

dissuade new-members from posting as much until they have been assimilated

Deterring them from posting will ward off good people because they'll lose momentum or be annoyed, and will increase the proportion of thick-skinned and / or persistent types who can deal with the annoyance. Not all thick-skinned / persistent people are bad, some are leaders or are gifted with those abilities, but creating gauntlets of annoyance will increase the proportion of undesirable thick-skinned / persistent types like trolls, newbie debaters, etc.

Essentially, dissuasion IS filtering, so if you're going to filter, you may as well be conscious of it and use a method that is likely to attract the type that you want. My questionnaire would filter for people who like learning or don't mind looking things up. The karma system currently in place filters for trolls and debaters. Dissuading people from posting will exacerbate the effect of the karma system if it remains as-is. The combination of the two would may result in a hideous unintentional filter.

Filtering is only useful in that it retards the acquisition of new recruits slow enough to allow the old recruits to assimilate.

If done well, it would also encourage a higher proportion of people that are the right type, discouraging mainstream people who aren't genuinely interested in the culture from creating a new majority and taking over. Which is why i suggested the questionnaire that I did. That would select for people genuinely interested in rationality, most others won't take the time to fill out such a questionnaire.

"How do we convince new members to lurk until they're assimilated?"

I disagree, but it might be "How do we convince members to lurk until their assimilated without scaring any of them off