Thanks for all the feedback. The bearings were fine in the hub, as was the wiring. My initial thought was that the easiest entry path for water would be along the wiring into the stator; the wiring didn't seem particularly well shielded. The link to the Joule hub is interesting. One thing I noticed is that the magnets in the Shimano hub are much thinner, and metallic in colour, perhaps a different composition? I think one of the reasons they shattered is because they are so thin. If they'd just rusted it would have been no problem to clean up the hub and get it running again.

What are people's thoughts on greasing / oiling the innards of the replacement hub with a thin layer to try and prevent the same problem reoccurring? (I have no qualms opening the new hub and voiding warranty as I believe in the right to repair / improve). Normally I service regular Shimano hubs before putting into to service because of lack of grease / over tightening of bearings from the factory.