I had an earlier version of this implemented in Haskell but was too hard to maintain
for many reasons.
So how does it work? You add
NIX_COMMENT_UPDATER_START { json }
NIX_COMMENT_UPDATER_END
And if an update implementation feels responsible by looking at json it will return a string which can contain newlines which will be indented appropriately.
Eg see git/github implementations. So they will add name = “name-revision”; version = “…”; and src =… tags between the comments.
More implementations can be added over time.
It’s dead simple and works.
Enjoy and provide feedback 
What’s the process of making this a community project ?