As I commented in https://news.ycombinator.com/item?id=44150238, all you need is a way to express what is pending, which can be done using JSON key paths.

Of course, you could do it in-band, too:

    {"comments": {"state": "pending", "values": []}}

…at the cost of needing your data model to be explicit about it. But this has the benefit of being diffable, of course, so once the data is available, the diff is just the new state and the new values.