There's a fine line between black out dates and limiting award inventory, especially as more pax earn status and record numbers of QFF points are on the books.
This creates a situation where it becomes incrementally more difficult to redeem points for outright redemption. This problem is compounded with oneworld/partner airlines accessing inventory too.
As uncle Dave says "When customers lose faith in the ability to reasonably redeem for a flight - loyalty dissipates and all the branding, marketing and built up goodwill quickly fades into the abyss"
A more logical approach would be by using the internal scoring metrics for a customer and grant seats based on a hybrid mix of historical + future + potential banked revenue from that customer. For example Platinum member may request SYD-LAX in F to be released for them +1 and the member is seen as a vanilla platinum with 1M QFF points. Using the internal scoring plus future earning potental banked revenue, QF may can easily find out how many points this member has stashed away in Amex MR (perhaps in the millions) - and not releasing a seat basically seals the deal those points will never hit the QF system.
I know of one airline which expects you to purchase in flight duty free when you receive a free upgrade, and new systems they have now track these purchases by seat number on flight etc/linking back to your op-up. By making a purchase as a pax on an op-up you're essentially training the airline that you are rewarding them through additional ancillary revenue purchases when you are upgraded. This in turn increases your internal score with the airline.
Then in a sense you could build up this internal score within the airline/ffp and cash in on it when you request an award seat release, thus lowering your score. If it's too low - then requests for seat releases will always be declined.
Here's a fantastic article on how good airlines measure loyalty -
http://bit.ly/28VVLBx. PCV is soo 1999. It's all about big data intelligence these days.