I thought that custom_json ops didn't use the C++ variant type, but just a JSON string. There is a bug in the JSON parser, but not an exploitable one in the way it is used. But I haven't looked at that in detail, it would certainly be good to understand what sort of things somebody could insert using a custom_json operation.
RE: Steemd 0.20.6 bug - memory exhaustion when parsing malicious hello_message