I find that I often wish I could ask GHC to tell me "where did *that* value come from?" as I debug JHC. Since GHC obviously can track this information, it seems like it would be fairly simple to implement a way for it to report this information as instructed by a running program. Perhaps:

reportCssFor :: a -> b -> b

I wouldn't really have any desire to use this on thunks, since when I want to do this it is usually because I don't like the value (i.e. it isn't in the right normal form), and to know this I must already have evaluated it.