Thanks for sharing your real world experiences – great taste in guitars too. I asked a friend who runs an instrument sales and repair shop in Holland for his take on humidity damage and what he said chimes with your experience in Arizona:
The average humidity in Holland is 77% because most of Holland is below sea level, with water everywhere. The difference, of course, is temperature but we have no problems with humidity here.
Guitars don't mind high humidity. The killer is if it's too dry – then they crack. The first thing to fail if the guitar dries out is the binding, it will separate. The binding being plastic doesn't move but the wood shrinks.
So it seems that if high temperatures can be minimised, guitars that are reasonably well looked after should survive unscathed.
I've drifted off-topic a bit, as my post was about customs duty costs for instruments. Still, I am grateful for the advice on here, which has spanned from "should be fine " to "fuggedaboutit". Indeed, customs duties aside, how well this equipment will fare after an extended period in the tropics does focus the mind on whether the game is worth the candle.