Sorry, I don't have answer for you, but I am very interested in this topic.
My son is allergic to nickle, which is found in most stainless steel watches. We are looking at titanium watches for him right now.
As far as tungsten goes, I think mostly its sold with the name tungsten and is an alloy with varying percentages of tungsten and therefore will contain nickle and other metals and is not hypoallergenic.
The ceramic watches I've seen mostly have stainless steel backs (titanium too), metal between the links and on the clasps. Rhodium plating is hypoallergenic.
Hope someone here has some answers, will be really interesting to find out about the reps...great topic...