I was able to try it on a test server, the combination of row_to_json() and json_strip_nulls() worked exactly as I had hoped. Stripping nulls reduced the JSON file by over 50%. (The data I needed to export has around 900,000 rows, so it gets quite large.)
I've got a test file I can make available to app developers.
My next task is to find out if validating and importing a JSON file into a table is as easy as exporting a table in JSON turned out to be. Thanks for the help. --