Just added an experimental OPML output to Typed Threaded Discussion.
Try getting an example from this page
OPML isn't ideal as an XML version of TTD. Too many superfluous levels to manage the nesting of children in different types. I could have made types other
attributes, but I have no idea whether other outliners would treat them in an interesting way or even display them.
But OPML is at least a standard that other pieces of software understand. So may get some alchemy out of it.