Hi @bubke, to be honest I do not know. I'm not sure it's possible. As far as I know a custom json is a special type of operation used to register things like: follow, reblog etc
You can find all the operation types here: https://developers.steem.io/apidefinitions/#apidefinitions-condenser-api
This is what I could find with a quick look:
RE: Learn how to program with Python #1 - Solving Puzzles from Advent of Code 2018