I havent't found any mod neither a builtin feature. Would it be hard to protect single threads with a password?

I'm building a RPG Board at the moment and it would be a rather useful feature.

Yes.  It would be much better to use member group and protect on a per-board basis.  Board passwords aren't really "secure" (anyone can share them, and have no reason not to).

Yes, I know. That would have been another question of me. I'd like to have some kind of "private board". That is, one Board for every single member, that only the administrator group and the member can see.

I asked, because I'll need a place to talk with only two persons, perhaps three, but I'd like to also make the prtected threads visible to everybody at a later time. Well, I could use two or three membergroups for this, yes. This would just be a bit more of work

Members can add themselves into groups, so that wouldn't be that much work.  

Quoted from Justin

Members can add themselves into groups, so that wouldn't be that much work.  

Yeah. I'm already using this for other purposes

Looks like it's really really time to learn some perl after all (will be fun after being forced to learn php I think)