IIRC jwz (before he decamped from LJ completely) had a script (which he posted/linked to his LJ, about a year back) that he would run at the LJ admin interface to mark "old" posts (for some definition of old: 2-4 weeks, I think) as not accepting comments. Presumably with minor changes one could change it to limit who can post after some period of time and, eg, allow anon comments for a short period of time then lock down to LJ-handle only, or LJ-friends only (or screen those that aren't in one of those categories).

Sometimes I miss the old Internet where the problem was making sure that everything got through, not figuring out which 90% to reject out of hand :-(

Ewen