Unfortunately, solids do not appear to do anything when people touch them/are stabbed by them. Even getting the object stuck in has no effect (I was really bummed by this one... I wanted to make poison darts).
I can't imagine why it wouldn't be possible to make a "gas chamber" like Fullmoon suggested. However, since there are no beneficial syndromes, the benefit of this is debatable.
...unless, of course, you just want another devious way of executing certain dwarves.
It is quite possible to have syndromes target specific castes. However, you'd need to make a separate syndrome for each caste if you wanted them to develop differing syndromes.
In other news, ENTERS_BLOOD does not appear to make afflicted blood contagious. So much for spreading disease...
An interesting note: Clothing and armor will become contaminated before you do. If you are dealing with a contact poison, a good layer of clothing can prevent you from getting infected. This is helpful if you happen to be wading through puddles of infectious goo for some reason (if you're fully covered, you can even roll around in the stuff).
However, being engulfed in a vapor will penetrate whatever clothing you've got and cover you from top to toe. Swimming will also cause everything to get wet, but this generally isn't a problem (magma protection test results are... Inconclusive).
I'm going to run a couple tests, regarding secretions and water contaminants (also dusting, but that's just because I can't remember testing it from before). I'll be back in a while.