I've been giving some consideration to a coating on the turbo for a while now. I think the standard ceramic coating you see on exhaust is too thick and not uniform enough for a tight tolerance wear surface. However when I was drag racing there was a ceramic coating some people were applying to the crank journals for increased lubricity. I need to do some research on that product to see what the nominal thickness is.
I think if you were to coat the housing and the ring, or replace the ring with a SS one it would definitely alleviate rusting and possibly due to the smooth surfaces would decrease adherence of carbon.
edit: Here's a site I found with some info on dry lubricant ceramic coating
CIC Crankshaft coatings
On a side note, here is an article that is written way better than I could say it in regards to why you would want to coat any part.
http://www.enginebuildermag.com/Article/2469/why_parts_get_coated.aspx
I think if you were to coat the housing and the ring, or replace the ring with a SS one it would definitely alleviate rusting and possibly due to the smooth surfaces would decrease adherence of carbon.
edit: Here's a site I found with some info on dry lubricant ceramic coating
CIC Crankshaft coatings
On a side note, here is an article that is written way better than I could say it in regards to why you would want to coat any part.
http://www.enginebuildermag.com/Article/2469/why_parts_get_coated.aspx