Like many, I tend to be a cynic these days...
With the reserve rest "look back" rule and some in house oversight, it "could" work with acceptable margins of safety.
That's a good point about shorter duty days with no sits at out stations, hadn't thought of that. At carrier "x", there was a rule that the duty day was a function of home domicile start time. If you released brakes at 0300 (for example) body clock time, then the duty was reduced. I recall that duty day was reduced 6 minutes for each minute prior to 0600 body clock time that the brakes were released.
That might be a good tool with the 8 hour waiver to ensure the waiver is an efficiency tool, and not a manpower tool. For every minute of 8 hours of flight time - max duty day is also reduced.
NASA Ames has lots ands lots of scientific data on sleep debt that they share for free. Might be a good resource:
http://human-factors.arc.nasa.gov/zteam/
Cheers