I don't know about the XML API but writing a TAP, or any app, that not only downloads the guide data but also downloads the schedule list was possible when we had the My Week RSS feed.
When the website was updated the handy My Week RSS Feed vanished.
Obviously there would not have been any two way syncing (no timer status updates sent to your account) but being able to download your week's recording list in a text format was a godsend.