We did get a hint this might sometimes be possible, we were booked
on the first available L seats on the famously busy SCL-ALK route and
wished to bring back the date by a few weeks, many days showed
plenty of Y seats in both the LA and QF allocations. The agent initally
said L was full but when we asked. Pointing out we had OW Saph and
QF status they went off to check if something was possible.
The answer from both LA and QF was that the route is too busy
and that is wasn't possible to get one of the Y seats. When we
booked SCL-AKL the first available L seats were 3 months away,
the situation seems similar now.

We also tried wait-listing for the LA seats, QF wouldn't waitlist but LA
would, however LA seemed to treat wait-listing like normal standby and
said we'd only know about 3hrs before departure whether seats
were available. We declined the wait-listing and used our time waiting for
the SCL flight to fly with LA to MEX instead.