I don't think he could do that. If he was seen to muck in then for any other natural disaster he'd have to appear and help, otherwise it's thumbing his nose at the ones he can't do.
If, on the other hand, he had pledged support for a charity that was about helping recovery of the Earthquake, then he could appear in full flight gear offering genuine help. At the moment it is a state sponsored visit, not a 'Wills' sponsored visit.