This may not be what you're talking about in the docs, but I would like to be able to use type converters, especially for managing things like IDs, which I typically wrap in a single-case union.
I have a thin SQL wrapper I wrote that lets me add converter functions to automatically serialize/deserialize when I run a query. This is especially useful with SQLite's much lighter type system. If I could do something like that in Rezoom.SQL, I would immediately jump ship and never look back.