Interesting. I suggest you add defsop to the arc-wiki.git
As for the process that does the archiving... if I remember correctly hacking through srv.arc shows that it will summarily kill any operations that take too long.
Well, concerning long threads... That's the reason why I fix threadlimit* at the beginning to 300 s. That's enough for archives 5OO MB long, and anyway, for even bigger archives, I wouldn't propose people to use this method...