I'm keeping this message for future review. Can't quite get my head around the workings quite yet, but what I'm coming up with so far looks really darned tough from this end, and is a bit akin to a threaded display format, which has its own problems.
However, somewhere around here I've got a largely-completed two-level threading system I wrote a while back and that just might be easily adaptable to something like this.
Can't really put any time into it at the moment, but I'll revisit this idea when I can and see what I think of it overall and what I can come up with if I decide to implement it.