Platinum U request - back into inventory?

This really can only be answered by our friends in Qantas "Yield Management"; that wonderful department of statistical revenue maximization.

However, given that a seat request was necessary in the first place, it is highly unlikely that it would be returned to U class. That does not imply it won't happen, rather that generally, it won't.

You never really know; I recently monitered a flight's fare bucket availability while booking an award seat in WHY. Once completed, not only did T reduce by one, so did the N, O, Q & X fare buckets. Other times it's been just T & X.