I did the tour in the Australian Alps you linked to in the first thread, Uncle Dave, so this isn't really a new source of info. Thanks for linking to it. I hope you enjoyed reading it.
The tour wasn't as long as that which you are proposing, but it was through some really tough country. The xtracycle handled really well throughout it all, and was a pleasure to ride.
If you use waterproof stuff sacks as for kayaking, the fact the 'panniers' are open doesn't present problems. Most people recommend that these are used inside waterproof panniers as well, so I don't see that as a drawback. It is easy to be well organised if you have multiple of these bags. The snap deck on the top is also useful for tying down anything you could think of. As long as things are packed well, you shouldn't have anything fall out.
The length of the wheelbase would make it good for corrugated roads. You get a really nice smooth ride. Dust getting in to the frame can be overcome by sealing the joins with the fingers of latex gloves or sections of a 700c innertube.
It is a valid point about chain tension- I used a tensioner and this worked well. There are a few ideas on the xtracycle support forum about other options to increase the tension.
I also use the xtra for around town, and it really is incredibly versatile. It will carry big loads and will handle well.
There is a small amount of wear on the chainbridge stay where the frame joins with the xtra. I don't really know what sort of long term problems could be created by this. At the moment it appears to be limited to scraping of paint off the frame.
I would say go for it. I own both a bob-trailer (my mate used that on our Snowy Trip) and the xtra. I find the xtra is more versatile, has a better carrying capacity/setup and has a smoother ride.