They already have (Legendary). It comes sometime before 1000 activity (slightly randomized for each user).
Yes you are right! Is it applied manually by Theymos or is there any algorithm behind it for random choosing?
Actually, this is a very interesting question. I too wonder.
It's an algorithm.
Currently this randomness is done with a single SQL query, which is very convenient.
update smf_members set ID_POST_GROUP=21 where ID_POST_GROUP=8 and
activity>=775 and activity>=775+conv(substr(sha1(concat(ID_MEMBER,
secretSeed)), 1, 2), 16, 10);