Dear Peter,
Thank you for your query.
I found some mentions of the ddxml integration that you refer to ten years ago:
- Trane Trace 700 GBXML Integration
- http://autocadmep.blogspot.com/2008/05/trane-trace-700-gbxml-integration.html
- https://autocadmep.wordpress.com/2008/05/30/trane-trace-700-gbxml-integration
I doubt that this export will be integrated natively into Revit or its API.
If you would like that to happen, you should submit a wish list item for it in the Revit Idea Station and gather lots of votes for it.
A different approach might provide faster and more fruitful:
Quite probably, all of the information defined in this schema is available through the Revit API.
You can use RevitLookup to test that assumption:
https://github.com/jeremytammik/RevitLookup
It implements an interactive Revit BIM database exploration tool to view and navigate element properties and relationships.
Every data item it displays has been retrieved via the Revit API.
If all the information required by ddxml is available through RevitLookup, then you can implement your own Revit add-in to perform the desired export.
I hope this helps.
Best regards,
Jeremy