Not nested, but perhaps chained is the keyword. You can extract something with JSONPATH and pass the result to a JS transformation.
I can’t see that’s very helpful; it sounds more like you want to pass the whole JSON to a JS script (which can deal with JSON natively), check the date element against “today”, and pass the rest of the JSON data along?
EDIT - I suppose chaing might be useful, after all. Each channel can only handle a single data item - but you could have multile channels, each using the same date-checking JS, but chained to different JSONPATH to extract each item.