[xfs]

> It's hard to picture what a 'typical' query for some field over a JSON document would look like.

Exactly. A "query" would have to define not only the path, type of the field in the source data but also the type/interface of where you want to put that data. Combining dynamic queries and typed data you get a fairly tricky problem, which is why I said this is tricky. I worked on a similar thing for protobuf and jitting was a solution I looked into (in that project libllvm was too unwieldy to use).