KGI has just removed the "A332" from the parking bay details in Jepp...
...and to have it put back again? Probably not a great effort required to have wide bodies included again if they already had been for years, I would have thought.
Kal is less than 60 Mins away. Its close enough. As long as Customs can be happy with inbound international services doing Gas and Goes where pax stay on the Aircraft, rather than wanting to meet aircraft at their first point of landing, the terminal facilities shouldn't be a barrier either. Kal and Gero (even closer- and probably already has Customs in town thanks to the Port) should do just fine so long as the usual issues (parking, lighting, etc) are covered.
Thats also not to mention the ability to carry Learmonth (-yes lighting issues at night, I know...), and possibly even Adelaide, on the LHR to PER sector is enhanced by the faster flight time in that direction.
Sure, there are issues to be dealt with before hand. However, for the relatively small amount of times a year that a diversion might be a consideration, I don't think the availability of alternates is going to be a massive barrier to it all. Regardless, it's certainly not the issue that warrants Darwin over Perth.