That's insane. Here I was thinking about how much more challenging it would be to parse and reconstruct the object from jq, and you got the idea to take advantage of the syntax similarity to parse it as jq code itself. Nice. And so, that means the inverse of the jq code I posted would simply be:

  ( jq "$(sed 's/$/ |/;$a.')" <<< '{}' )

As in:

  catj example.json \
  | ( jq "$(sed 's/$/ |/;$a.')" <<< '{}' ) \
  > original.json