among highly rational peers

Tricky (like most anything).

I wouldn't say "among rational peers" so much as "among EA-oriented peers". For our specific community, there is significant overlap in the Venn diagram depicting those two qualities, but those two are very much distinct qualities nonetheless.

A community of HPMOR!Quirrell variations would have your very post in main, with plenty of upvotes, all the while secretly whetting their blades. Perfectly rational.

The more established the trust culture, the more vulnerable it would be to a traitor, a cunning red-pill bastard who plays the trust-network like a fiddle to the tune of his/her egotistical agenda.

Trust -- the quintessential element of your so-called "tell culture" -- and vulnerability are two sides of the same coin.

When the social circle is small enough as to resemble an expanded family unit, a clan, it may work. A strong sense of ties that bind to keep the commitment to honesty honest would tend to keep a "tell culture"' social circle's cardinality well below Dunbar's number.