interesting issue Mev (although of course, not from your perspective) I ride pretty regularly, mostly commuting, and havent done any trips over about a month in length, but I have never in my life had a hub go like that.
Granted, I'm a light guy with rarely more than 45lbs of a load, but I don't see how load would change the issue here--I really do wonder if its a contamination issue, I know you have done pretty tough trips, and am doing, where dirt, mud etc is on a whole diff scale than my riding and touring experiences.
Given your past failures, and specifically the pawls going wonky in some way, it reminds of a guy who recently told me of winter riding and how he has had pawls do this from the cold, and banging his wheel to get the pawls to engage again--which brings me back to the contamination idea of stuff getting gunked up royally in your cases??
who knows, not much of a help to your question, just throwing out some ideas that could perhaps help reduce the risk of it happening again, if certain hubs are known to have much better seals etc.
here are some questions though--am I right in thinking that your bike possibly gets really mudded up a lot (Im thinking of your Alaska dirt road bit you dealt with, I know you mentioned it was muddy and tough)
wish I had some proper suggestions other than just ideas of "why"
All the best with finding hubs that last for you.