It's not about flights coming from the east coast. They're never really limited in terms of range. There are plenty of options available on the way if it gets bad.
This is about ultra long haul flights coming in from the north west. Particularly LHR-PER.
Cunderdin makes sense for this as the extra fuel uplift is negligible, it could be taken care of in the descent with an extra 10 mins or so of cruise.
It'll be interesting though if a wide body got stuck out there. No accomodation, no customs. Everybody remains onboard until a replacement crew can be sourced and paxed out. *shudder*
I think it will be a planning exercise only. Just like Forrest. With the cat3b in PER, if there is a real diversion it'll go somewhere more suitable for a recovery.