Very much interested in this here too!
Meanwhile, having just read through this thread and noticed some helpful suggestions I've not stumbled across before, I'm playing around now with a combination of the 'DIY marker track' (colourful dummy clips in a dedicated silent MIDI track) and clip groups to get a pseudo version of this going. It actually works pretty well, though there are limitations - it doesn't have auto-ripple features, can't auto-select any automation that isn't in a clip, and it's obviously more ponderous to move things around manually on the timeline than to be able to just whip up a playlist of sections and check it out, maybe even dropping things in on the fly as you go...
However, that is all quite possible in Matrix - I know that's another kettle of fish, and not necessarily perfectly adapted to creating song sections, which is often kind of a linear experience for many of us that doesn't necessarily transpose to the pattern-based mindset of the Matrix... BUT, having created the sections and clip groups in the timeline, it's rather simple to drop them into Matrix channels and then there they are, ready to go, and improvised playlists can at least be recorded in real time to empty spots on the timeline. (Automation sadly seems not to be handled at all in the Matrix, as far as I can tell, maybe someone can correct me here?)
Not the same as is being asked for, obviously, and I hope we can get the full meal deal at some point, but still... while we wait to see if/when this much-requested and apparently long-considered feature comes to fruition, this is something I can use now to rough out song structure without manually selecting and moving things every time. So that's progress! However, a full-fledged arrange feature would be more progress. Looking forward to seeing this soon... I hope...