Julien,
If Cake would allow certain members to be somewhat of a forum mod it would have to be for the sole purpose of deleting obvious spam only (such as the kitchen adds or selling shoes).
Not to moderate any other thread in any other way.
The 3-5 member idea is a good idea I think.
If I was a 'mod' I would open the thread and choose delete. Then as the other 'mod's' see it they would do the same. If 3 or 4 out of 5 mark it, it get's deleted.
I also can tell a spam post usually from the subject alone. Those I just pass over or
go in and report it. 98% of the time another forumite has already reported it and Cake is quick to flush it.. The only time spam post's hang around for more than one day is on weekends when no one is in the office to kill it.
Cake actually does a good job with dealing with spammers.
Mike