Check out
this thread - we've been discussing this recently. We don't know enough yet to theorize with any sort of accuracy, though.
It's possible (based on the 990's design) that zone 2 and zone 3 are purely analog pre-amps, in which case it doesn't seem possible that the 997 could offer a 9.1 setup. It seems more plausible that PLIIz could be implemented so that users could choose from either a 7.1 setup with rear surrounds
or a 7.1 setup that offered the standard 5.1 plus the two height channels (using the rear surround pre-amp outputs). That's
pure speculation, though.
At this point, I think the one thing we can safely say is that there's about a 99.92% chance that PLIIz will not be part of the feature list when the 997 launches.