Amazon Ion has some support for this - items are length-prefixed so you can skip over them easily.

It falls down if you have e.g. an array of 1 million small items, because you still need to skip over 999999 items to get to the last one. It looks like RX adds some support for indexes to improve that.

I was in this situation where we needed to sparsely read huge JSON files. In the end we just switched to SQLite which handles all that perfectly. I'd probably still use it over RX, even though there's a somewhat awkward impedance mismatch between SQL and structs.