I don’t know DevonThink that well… but in the post you made in there forum there was some mention of importing OPML into a file/folder hierarchy in devonthing. The birch-outline project would make that sort of thing (but with task paper files) very easy.
On the other hand if you are talking about embedding the TaskPaper editor in devonthink that would be much harder. I’ve open sourced TaskPaper’s model layer, but not the editing layer. If you wanted to recreate a TaskPaper like editor the model layer would be a good starting point, but there would still be lots of other work to do. I don’t know, but I think it would be unlikely that devonthink would want to do that part.