Following up on this, does anyone have a direct experience of what happens in this situation?
I can currently upgrade the shorter of two return segments on a booking - it's the longer one that I want to auto-upgrade. I could upgrade the shorter segment and keep my fingers crossed for the longer one, but the shorter segment alone is a waste of miles.
If no one else has experience of this, I'll report back if/when it happens for me.
Have the return from MRU next week on auto upgrade and waitlisted - will let you know.